Portable backward compatibility with MacOS 10.12+ (#7016)

Summary:
When `PORTABLE=1` is set, RocksDB will now be built with backwards compatibility for MacOS as far back as 10.12 (i.e. 2016).
Pull Request resolved: https://github.com/facebook/rocksdb/pull/7016

Reviewed By: ajkr

Differential Revision: D22211312

Pulled By: pdillinger

fbshipit-source-id: 7b0858d9b55d6265d3ea27bf5ea1673639b6538c

build_tools/build_detect_platform

@@ -623,6 +623,13 @@ else
   if test "$USE_SSE"; then
     TRY_SSE_ETC="1"
   fi
+
+  if [[ "${PLATFORM}" == "OS_MACOSX" ]]; then
+    # For portability compile for macOS 10.12 (2016) or newer
+    COMMON_FLAGS="$COMMON_FLAGS -mmacosx-version-min=10.12"
+    PLATFORM_LDFLAGS="$PLATFORM_LDFLAGS -mmacosx-version-min=10.12"
+    PLATFORM_SHARED_LDFLAGS="$PLATFORM_SHARED_LDFLAGS -mmacosx-version-min=10.12"
+  fi
 fi
 
 if test "$TRY_SSE_ETC"; then