See https://bugzilla.mozilla.org/591152

diff -up mozilla-central/configure.in.orig mozilla-central/configure.in
--- mozilla-central/configure.in.orig	2010-09-14 21:41:14.883579761 +0200
+++ mozilla-central/configure.in	2010-09-14 21:42:23.976579149 +0200
@@ -3787,7 +3787,7 @@ fi
 
 dnl check for wcrtomb/mbrtowc
 dnl =======================================================================
-if test -z "$MACOS_DEPLOYMENT_TARGET" -o "$MACOS_DEPLOYMENT_TARGET" -ge "100300"; then
+if test "${MACOS_DEPLOYMENT_TARGET:-999999}" -ge "100300"; then
 AC_LANG_SAVE
 AC_LANG_CPLUSPLUS
 AC_CACHE_CHECK(for wcrtomb,
diff -up mozilla-central/configure.orig mozilla-central/configure
--- mozilla-central/configure.orig	2010-09-14 21:41:09.557839675 +0200
+++ mozilla-central/configure	2010-09-14 21:42:10.369954678 +0200
@@ -11978,7 +11978,7 @@ EOF
     
 fi
 
-if test -z "$MACOS_DEPLOYMENT_TARGET" -o "$MACOS_DEPLOYMENT_TARGET" -ge "100300"; then
+if test "${MACOS_DEPLOYMENT_TARGET:-999999}" -ge "100300"; then
 
 ac_ext=C
 # CXXFLAGS is not in ac_cpp because -g, -O, etc. are not valid cpp options.
diff -up mozilla-central/js/src/configure.in.orig mozilla-central/js/src/configure.in
--- mozilla-central/js/src/configure.in.orig	2010-09-14 21:41:24.605579748 +0200
+++ mozilla-central/js/src/configure.in	2010-09-14 21:42:49.229954365 +0200
@@ -3309,7 +3309,7 @@ fi
 
 dnl check for wcrtomb/mbrtowc
 dnl =======================================================================
-if test -z "$MACOS_DEPLOYMENT_TARGET" -o "$MACOS_DEPLOYMENT_TARGET" -ge "100300"; then
+if test "${MACOS_DEPLOYMENT_TARGET:-999999}" -ge "100300"; then
 AC_LANG_SAVE
 AC_LANG_CPLUSPLUS
 AC_CACHE_CHECK(for wcrtomb,
diff -up mozilla-central/js/src/configure.orig mozilla-central/js/src/configure
--- mozilla-central/js/src/configure.orig	2010-09-14 21:41:20.036829607 +0200
+++ mozilla-central/js/src/configure	2010-09-14 21:42:42.767876525 +0200
@@ -10234,7 +10234,7 @@ EOF
 
 fi
 
-if test -z "$MACOS_DEPLOYMENT_TARGET" -o "$MACOS_DEPLOYMENT_TARGET" -ge "100300"; then
+if test "${MACOS_DEPLOYMENT_TARGET:-999999}" -ge "100300"; then
 
 ac_ext=C
 # CXXFLAGS is not in ac_cpp because -g, -O, etc. are not valid cpp options.