compile with correct flags to determine SSE4.2 support

Summary:
With some compilers, `-std=c++11` is necessary for <cstdint> to be
available. Pass this flag via $PLATFORM_CXXFLAGS. Fixes #2488.
Closes https://github.com/facebook/rocksdb/pull/2545

Differential Revision: D5620610

Pulled By: yiwu-arbug

fbshipit-source-id: 2f975b8c1ad52e283e677d9a33543abd064f13ce

build_tools/build_detect_platform

@@ -458,7 +458,7 @@ elif test -z "$PORTABLE"; then
   fi
 fi
 
-$CXX $COMMON_FLAGS -x c++ - -o /dev/null 2>/dev/null <<EOF
+$CXX $PLATFORM_CXXFLAGS $COMMON_FLAGS -x c++ - -o /dev/null 2>/dev/null <<EOF
   #include <cstdint>
   #include <nmmintrin.h>
   int main() {