Only check for & use -fshort-wchar with CXX as some versions of gcc only support -fshort-wchar for g++.

Bug #139211 r=bryner

configure.in

@@ -1519,13 +1519,15 @@ if test "$ac_cv_have_usable_wchar" = "yes"; then
     HAVE_CPP_2BYTE_WCHAR_T=1
 else
 dnl This is really gcc-only
-dnl Do this test using CXX instead of CC since some versions of gcc
-dnl 2.96-2.97 have a signed wchar_t in c++ only.
+dnl Do this test using CXX only since some versions of gcc
+dnl 2.95-2.97 have a signed wchar_t in c++ only and some versions
+dnl only have short-wchar support for c++.
+dnl Note that we assume that mac & win32 have short wchar (see nscore.h)
 
-    _SAVE_CFLAGS=$CFLAGS
-    _SAVE_CC=$CC
-    CC=$CXX
-    CFLAGS="$CFLAGS -fshort-wchar"
+    AC_LANG_SAVE
+    AC_LANG_CPLUSPLUS
+    _SAVE_CXXFLAGS=$CXXFLAGS
+    CXXFLAGS="$CXXFLAGS -fshort-wchar"
 
     AC_CACHE_CHECK(for compiler -fshort-wchar option, 
         ac_cv_have_usable_wchar_option,
@@ -1540,12 +1542,10 @@ dnl 2.96-2.97 have a signed wchar_t in c++ only.
     if test "$ac_cv_have_usable_wchar_option" = "yes"; then
         AC_DEFINE(HAVE_CPP_2BYTE_WCHAR_T)
         HAVE_CPP_2BYTE_WCHAR_T=1
-dnl Since this is gcc-only, CC and CXX should take the same arguments.
-        CXXFLAGS="$CXXFLAGS -fshort-wchar"
     else    
-        CFLAGS=$_SAVE_CFLAGS
+        CXXFLAGS=$_SAVE_CXXFLAGS
     fi
-    CC=$_SAVE_CC
+    AC_LANG_RESTORE
 fi
 
 dnl Checks for header files.