Enforce c+11 on OSX

Recommendation of StackOverflow - https://stackoverflow.com/questions/45047508/error-unknown-type-name-constexpr-during-make-in-mac-os-x

This will allow the snappy include to be detected, otherwise it will fail due to:

... eleveldb/c_src/system/include/snappy.h:199:10: error: unknown type name 'constexpr'
  static constexpr int kBlockLog = 16;

Also required in COMMON_FLAGS so that leveldb will compile with snappy

build_detect_platform

@@ -68,7 +68,8 @@ case "$TARGET_OS" in
         IFS="$oIFS"
         if [ "$1" -ge 13 ]; then
             # assume clang compiler
-            COMMON_FLAGS="-mmacosx-version-min=10.8 -DOS_MACOSX -stdlib=libc++"
+            COMMON_FLAGS="-mmacosx-version-min=10.8 -DOS_MACOSX -stdlib=libc++ -std=c++11"
+            CFLAGS="$CFLAGS -std=c++11"
             PLATFORM_LDFLAGS="-mmacosx-version-min=10.8"
         else
             COMMON_FLAGS="-fno-builtin-memcmp -DOS_MACOSX"