--- a/build/unix/elfhack/Makefile.in
+++ b/build/unix/elfhack/Makefile.in
@@ -11,16 +11,18 @@ NO_PROFILE_GUIDED_OPTIMIZE = 1
 VPATH += $(topsrcdir)/build
 
 OS_CXXFLAGS := $(filter-out -fno-exceptions,$(OS_CXXFLAGS)) -fexceptions
 
 WRAP_LDFLAGS=
 
 include $(topsrcdir)/config/rules.mk
 
+DEFINES += -DELFHACK_BUILD
+
 test-array$(DLL_SUFFIX) test-ctors$(DLL_SUFFIX): %$(DLL_SUFFIX): %.$(OBJ_SUFFIX) elfhack $(filter inject/%,$(CSRCS:.c=.$(OBJ_SUFFIX)))
 	$(MKSHLIB) $(LDFLAGS) $< -nostartfiles
 	@echo ===
 	@echo === If you get failures below, please file a bug describing the error
 	@echo === and your environment \(compiler and linker versions\), and use
 	@echo === --disable-elf-hack until this is fixed.
 	@echo ===
 	# Fail if the library doesn't have $(DT_TYPE) .dynamic info

--- a/build/unix/elfhack/inject/Makefile.in
+++ b/build/unix/elfhack/inject/Makefile.in
@@ -25,10 +25,12 @@ include $(topsrcdir)/config/rules.mk
 
 export:: $(CSRCS:.c=.$(OBJ_SUFFIX))
 
 $(CSRCS): %.c: ../inject.c
 	cp $< $@
 
 GARBAGE += $(CSRCS)
 
+DEFINES += -DELFHACK_BUILD
+
 CFLAGS := -O2 -fno-stack-protector $(filter -m% -I%,$(CFLAGS))
 $(CPU)-noinit.$(OBJ_SUFFIX): DEFINES += -DNOINIT

--- a/config/Makefile.in
+++ b/config/Makefile.in
@@ -123,17 +123,17 @@ install::
 	$(SYSINSTALL) $(IFLAGS1) $(DEPTH)/mozilla-config.h $(DESTDIR)$(includedir)
 
 GARBAGE += \
   $(FINAL_LINK_COMPS) $(FINAL_LINK_LIBS) $(FINAL_LINK_COMP_NAMES) buildid $(srcdir)/*.pyc *.pyc
 
 ifndef CROSS_COMPILE
 ifdef USE_ELF_DYNSTR_GC
 elf-dynstr-gc: elf-dynstr-gc.c $(GLOBAL_DEPS) $(call mkdir_deps,$(MDDEPDIR))
-	$(CC) $(COMPILE_CFLAGS) $(GLIB_CFLAGS) -o $@ $< $(LDFLAGS) $(GLIB_LIBS)
+	$(CC) $(COMPILE_CFLAGS) $(GLIB_CFLAGS) -DELFDYNSTRGC_BUILD -o $@ $< $(LDFLAGS) $(GLIB_LIBS)
 endif
 endif
 
 FORCE:
 
 check-preqs = \
   check-jar-mn \
   check-makefiles \

--- a/js/public/TypeDecls.h
+++ b/js/public/TypeDecls.h
@@ -45,21 +45,17 @@ class JSString;
 #endif
 
 #ifdef JS_USE_JSID_STRUCT_TYPES
 struct jsid;
 #else
 typedef ptrdiff_t jsid;
 #endif
 
-#ifdef WIN32
-typedef wchar_t  jschar;
-#else
-typedef uint16_t jschar;
-#endif
+typedef char16_t jschar;
 
 namespace JS {
 
 class Value;
 template <typename T> class Handle;
 template <typename T> class MutableHandle;
 
 typedef Handle<JSFunction*> HandleFunction;

--- a/js/src/js-confdefs.h.in
+++ b/js/src/js-confdefs.h.in
@@ -5,9 +5,11 @@
 
 #ifndef js_confdefs_h
 #define js_confdefs_h
 
 @ALLDEFINES@
 
 #include "js/RequiredDefines.h"
 
+#include "mozilla/Char16.h"
+
 #endif /* js_confdefs_h */

--- a/mfbt/Char16.h
+++ b/mfbt/Char16.h
@@ -9,19 +9,16 @@
 #ifndef mozilla_Char16_h
 #define mozilla_Char16_h
 
 /*
  * C11 and C++11 introduce a char16_t type and support for UTF-16 string and
  * character literals. C++11's char16_t is a distinct builtin type. C11's
  * char16_t is a typedef for uint_least16_t. Technically, char16_t is a 16-bit
  * code unit of a Unicode code point, not a "character".
- *
- * For now, Char16.h only supports C++ because we don't want mix different C
- * and C++ definitions of char16_t in the same code base.
  */
 
 #ifdef _MSC_VER
    /*
     * C++11 says char16_t is a distinct builtin type, but Windows's yvals.h
     * typedefs char16_t as an unsigned short. We would like to alias char16_t
     * to Windows's 16-bit wchar_t so we can declare UTF-16 literals as constant
     * expressions (and pass char16_t pointers to Windows APIs). We #define our
@@ -34,28 +31,47 @@
       (__cplusplus >= 201103L || defined(__GXX_EXPERIMENTAL_CXX0X__))
    /* C++11 has a builtin char16_t type. */
 #  define MOZ_UTF16_HELPER(s) u##s
    /**
     * This macro is used to distinguish when char16_t would be a distinct
     * typedef from wchar_t.
     */
 #  define MOZ_CHAR16_IS_NOT_WCHAR
+#elif !defined(__cplusplus)
+#  if defined(WIN32)
+#    include <yvals.h>
+     typedef wchar_t char16_t;
+#  else
+     /**
+      * We can't use the stdint.h uint16_t type here because including
+      * stdint.h will break building some of our C libraries, such as
+      * sqlite.
+      */
+     typedef unsigned short char16_t;
+#  endif
 #else
 #  error "Char16.h requires C++11 (or something like it) for UTF-16 support."
 #endif
 
+/* This is a temporary hack until bug 927728 is fixed. */
+#define __PRUNICHAR__
+typedef char16_t PRUnichar;
+
 /*
  * Macro arguments used in concatenation or stringification won't be expanded.
  * Therefore, in order for |MOZ_UTF16(FOO)| to work as expected (which is to
  * expand |FOO| before doing whatever |MOZ_UTF16| needs to do to it) a helper
  * macro, |MOZ_UTF16_HELPER| needs to be inserted in between to allow the macro
  * argument to expand. See "3.10.6 Separate Expansion of Macro Arguments" of the
  * CPP manual for a more accurate and precise explanation.
  */
 #define MOZ_UTF16(s) MOZ_UTF16_HELPER(s)
 
+#if defined(__cplusplus) && \
+    (__cplusplus >= 201103L || defined(__GXX_EXPERIMENTAL_CXX0X__))
 static_assert(sizeof(char16_t) == 2, "Is char16_t type 16 bits?");
 static_assert(char16_t(-1) > char16_t(0), "Is char16_t type unsigned?");
 static_assert(sizeof(MOZ_UTF16('A')) == 2, "Is char literal 16 bits?");
 static_assert(sizeof(MOZ_UTF16("")[0]) == 2, "Is string char 16 bits?");
+#endif
 
 #endif /* mozilla_Char16_h */

--- a/mozilla-config.h.in
+++ b/mozilla-config.h.in
@@ -21,16 +21,29 @@
  */
 #define __STDC_LIMIT_MACROS
 #define __STDC_CONSTANT_MACROS
 #if !defined(__STDC_FORMAT_MACROS)
 #define __STDC_FORMAT_MACROS
 #endif
 
 /*
+ * Force-include Char16.h in order to define PRUnichar as char16_t everywhere.
+ * Note that this should be the first #include to make sure that prtypes.h does
+ * not attempt to define PRUnichar.  This includes the following hunspell-specific
+ * includes.
+ *
+ * We don't use this to build elfhack and elf-dynstr-gc since those builds happen
+ * during the export tier.  Also, disable this when building assembly files too.
+ */
+#if !defined(ELFHACK_BUILD) && !defined(ELFDYNSTRGC_BUILD) && !defined(__ASSEMBLER__)
+#include "mozilla/Char16.h"
+#endif
+
+/*
  * Force-include hunspell_alloc_hooks.h and hunspell_fopen_hooks.h for hunspell,
  * so that we don't need to modify them directly.
  *
  * HUNSPELL_STATIC is defined in extensions/spellcheck/hunspell/src/Makefile.in,
  * unless --enable-system-hunspell is defined.
  */
 #if defined(HUNSPELL_STATIC)
 #include "hunspell_alloc_hooks.h"

--- a/xpcom/base/nscore.h
+++ b/xpcom/base/nscore.h
@@ -320,28 +320,16 @@
  * IUnknown.
  */
 #ifdef XP_WIN
 typedef unsigned long nsrefcnt;
 #else
 typedef uint32_t nsrefcnt;
 #endif
 
-/* ------------------------------------------------------------------------ */
-/* Casting macros for hiding C++ features from older compilers */
-
-#ifndef __PRUNICHAR__
-#define __PRUNICHAR__
-  #if defined(WIN32)
-    typedef wchar_t PRUnichar;
-  #else
-    typedef uint16_t PRUnichar;
-  #endif
-#endif
-
 /*
  * Use these macros to do 64bit safe pointer conversions.
  */
 
 #define NS_PTR_TO_INT32(x)  ((int32_t)  (intptr_t) (x))
 #define NS_PTR_TO_UINT32(x) ((uint32_t) (intptr_t) (x))
 #define NS_INT32_TO_PTR(x)  ((void *)   (intptr_t) (x))