--- a/configure.in
+++ b/configure.in
@@ -11,19 +11,23 @@ dnl the --with-external-source-dir rules
 dnl ========================================================
 divert(0)dnl
 #!/bin/sh
 SRCDIR=$(dirname $0)
 TOPSRCDIR="$SRCDIR"
 MOZILLA_SRCDIR="${SRCDIR}/mozilla"
 export OLD_CONFIGURE="${MOZILLA_SRCDIR}"/old-configure
 
-# Ensure the comm-* values are used.
-export MOZ_SOURCE_CHANGESET=$(hg -R "$TOPSRCDIR" parent --template="{node}" 2>/dev/null)
-export MOZ_SOURCE_REPO=$(hg -R "$TOPSRCDIR" showconfig paths.default 2>/dev/null | sed -e "s/^ssh:/https:/")
+# Ensure the comm-* values are used unless overridden by environment variables.
+if test -z "$MOZ_SOURCE_CHANGESET"; then
+    export MOZ_SOURCE_CHANGESET=$(hg -R "$TOPSRCDIR" parent --template="{node}" 2>/dev/null)
+fi
+if test -z "$MOZ_SOURCE_REPO"; then
+    export MOZ_SOURCE_REPO=$(hg -R "$TOPSRCDIR" showconfig paths.default 2>/dev/null | sed -e "s/^ssh:/https:/")
+fi
 
 # If MOZCONFIG isn't set, use the .mozconfig from the current directory. This
 # overrides the lookup in mozilla-central's configure, which looks in the wrong
 # directory for this file.
 if test -z "$MOZCONFIG" -a -f "$SRCDIR"/.mozconfig; then
     export MOZCONFIG="$SRCDIR"/.mozconfig
 elif test -z "$MOZCONFIG" -a -f "$SRCDIR"/mozconfig; then
     export MOZCONFIG="$SRCDIR"/mozconfig