--- a/js/src/builtin/embedjs.py
+++ b/js/src/builtin/embedjs.py
@@ -138,17 +138,17 @@ def get_config_defines(buildconfig):
   return env
 
 def process_inputs(namespace, c_out, msg_file, inputs):
   deps = [path for path in inputs if path.endswith(".h") or path.endswith(".h.js")]
   sources = [path for path in inputs if path.endswith(".js") and not path.endswith(".h.js")]
   assert len(deps) + len(sources) == len(inputs)
   cxx = shlex.split(buildconfig.substs['CXX'])
   pp_option = buildconfig.substs['PREPROCESS_OPTION']
-  cppflags = shlex.split(buildconfig.substs['OS_CPPFLAGS'])
+  cppflags = buildconfig.substs['OS_CPPFLAGS']
   cppflags += shlex.split(buildconfig.substs['WARNINGS_AS_ERRORS'])
   env = get_config_defines(buildconfig)
   js_path = re.sub(r"\.out\.h$", "", c_out.name) + ".js"
   msgs = messages(msg_file)
   with open(js_path, 'w') as js_out:
     embed(cxx, pp_option, cppflags, msgs, sources, c_out, js_out, namespace, env)
 
 def generate_selfhosted(c_out, msg_file, *inputs):

--- a/js/src/old-configure.in
+++ b/js/src/old-configure.in
@@ -1464,20 +1464,20 @@ if test -n "$MOZ_OPTIMIZE"; then
     AC_MSG_RESULT([$_results])
     if test "$_results" = "no"; then
         AC_MSG_ERROR([These compiler flags are invalid: $MOZ_OPTIMIZE_FLAGS])
     fi
     CFLAGS=$_SAVE_CFLAGS
 fi
 fi # COMPILE_ENVIRONMENT
 
-AC_SUBST(MOZ_FRAMEPTR_FLAGS)
-AC_SUBST(MOZ_OPTIMIZE_FLAGS)
+AC_SUBST_LIST(MOZ_FRAMEPTR_FLAGS)
+AC_SUBST_LIST(MOZ_OPTIMIZE_FLAGS)
 AC_SUBST(MOZ_OPTIMIZE_LDFLAGS)
-AC_SUBST(MOZ_PGO_OPTIMIZE_FLAGS)
+AC_SUBST_LIST(MOZ_PGO_OPTIMIZE_FLAGS)
 
 dnl ========================================================
 dnl = Trace logging. Enabled by default in DEBUG builds.
 dnl ========================================================
 MOZ_ARG_ENABLE_BOOL(trace-logging,
 [  --enable-trace-logging   Enable trace logging],
     ENABLE_TRACE_LOGGING=1,
     ENABLE_TRACE_LOGGING= )
@@ -2015,19 +2015,19 @@ AC_SUBST(MOZ_SYSTEM_NSPR)
 
 OS_CFLAGS="$CFLAGS"
 OS_CXXFLAGS="$CXXFLAGS"
 OS_CPPFLAGS="$CPPFLAGS"
 OS_COMPILE_CFLAGS="$COMPILE_CFLAGS"
 OS_COMPILE_CXXFLAGS="$COMPILE_CXXFLAGS"
 OS_LDFLAGS="$LDFLAGS"
 OS_LIBS="$LIBS"
-AC_SUBST(OS_CFLAGS)
-AC_SUBST(OS_CXXFLAGS)
-AC_SUBST(OS_CPPFLAGS)
+AC_SUBST_LIST(OS_CFLAGS)
+AC_SUBST_LIST(OS_CXXFLAGS)
+AC_SUBST_LIST(OS_CPPFLAGS)
 AC_SUBST_LIST(OS_COMPILE_CFLAGS)
 AC_SUBST_LIST(OS_COMPILE_CXXFLAGS)
 AC_SUBST(OS_LDFLAGS)
 AC_SUBST(OS_LIBS)
 
 AC_SUBST(HOST_CC)
 AC_SUBST(HOST_CXX)
 AC_SUBST(HOST_CFLAGS)

--- a/old-configure.in
+++ b/old-configure.in
@@ -3734,20 +3734,20 @@ if test -n "$MOZ_OPTIMIZE"; then
     AC_MSG_RESULT([$_results])
     if test "$_results" = "no"; then
         AC_MSG_ERROR([These compiler flags for C are invalid: $MOZ_OPTIMIZE_FLAGS])
     fi
     CFLAGS=$_SAVE_CFLAGS
 fi
 fi # COMPILE_ENVIRONMENT
 
-AC_SUBST(MOZ_FRAMEPTR_FLAGS)
-AC_SUBST(MOZ_OPTIMIZE_FLAGS)
+AC_SUBST_LIST(MOZ_FRAMEPTR_FLAGS)
+AC_SUBST_LIST(MOZ_OPTIMIZE_FLAGS)
 AC_SUBST(MOZ_OPTIMIZE_LDFLAGS)
-AC_SUBST(MOZ_PGO_OPTIMIZE_FLAGS)
+AC_SUBST_LIST(MOZ_PGO_OPTIMIZE_FLAGS)
 
 dnl ========================================================
 dnl = Disable treating compiler warnings as errors
 dnl ========================================================
 if test -z "$MOZ_ENABLE_WARNINGS_AS_ERRORS"; then
    WARNINGS_AS_ERRORS=''
 fi
 
@@ -4811,19 +4811,19 @@ AC_SUBST(OS_COMPILE_CMMFLAGS)
 
 OS_CFLAGS="$CFLAGS"
 OS_CXXFLAGS="$CXXFLAGS"
 OS_CPPFLAGS="$CPPFLAGS"
 OS_COMPILE_CFLAGS="$COMPILE_CFLAGS"
 OS_COMPILE_CXXFLAGS="$COMPILE_CXXFLAGS"
 OS_LDFLAGS="$LDFLAGS"
 OS_LIBS="$LIBS"
-AC_SUBST(OS_CFLAGS)
-AC_SUBST(OS_CXXFLAGS)
-AC_SUBST(OS_CPPFLAGS)
+AC_SUBST_LIST(OS_CFLAGS)
+AC_SUBST_LIST(OS_CXXFLAGS)
+AC_SUBST_LIST(OS_CPPFLAGS)
 AC_SUBST_LIST(OS_COMPILE_CFLAGS)
 AC_SUBST_LIST(OS_COMPILE_CXXFLAGS)
 AC_SUBST(OS_LDFLAGS)
 AC_SUBST(OS_LIBS)
 
 AC_SUBST(HOST_CC)
 AC_SUBST(HOST_CXX)
 AC_SUBST(HOST_CFLAGS)

--- a/toolkit/content/moz.build
+++ b/toolkit/content/moz.build
@@ -4,17 +4,17 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 TEST_DIRS += ['tests']
 
 for var in ('target', 'MOZ_CONFIGURE_OPTIONS', 'CC', 'CC_VERSION', 'CXX'):
     DEFINES[var] = CONFIG[var]
 
-DEFINES['CFLAGS'] = CONFIG['OS_CFLAGS']
+DEFINES['CFLAGS'] = ' '.join(CONFIG['OS_CFLAGS'])
 
 if CONFIG['OS_TARGET'] == 'Android':
     DEFINES['ANDROID_PACKAGE_NAME'] = CONFIG['ANDROID_PACKAGE_NAME']
     DEFINES['MOZ_USE_LIBCXX'] = True
 
 if CONFIG['MOZ_INSTALL_TRACKING']:
     DEFINES['MOZ_INSTALL_TRACKING'] = 1