Fix CXXFLAGS for huge_val.m4.

Patch by Jeremy Huddleston!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@153862 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/configure b/configure
index f4c5670..5c2d466 100755
--- a/configure
+++ b/configure
@@ -16371,7 +16371,7 @@
 ac_compiler_gnu=$ac_cv_cxx_compiler_gnu
 
     ac_save_CXXFLAGS=$CXXFLAGS
-    CXXFLAGS+=" -pedantic"
+    CXXFLAGS="$CXXFLAGS -pedantic"
     if test "$cross_compiling" = yes; then
   ac_cv_huge_val_sanity=yes
 else