Revert "[FB Internal] Remove code for FB-only toolchain (since GCC 4.8 is everywhere already)"

This reverts commit 61720850f6.

build_tools/build_detect_platform

@@ -46,6 +46,19 @@ PLATFORM_CXXFLAGS="-std=c++11"
 # we currently depend on POSIX platform
 COMMON_FLAGS="-DROCKSDB_PLATFORM_POSIX -DROCKSDB_LIB_IO_POSIX"
 
+# Default to fbcode gcc on internal fb machines
+if [ -z "$ROCKSDB_NO_FBCODE" -a -d /mnt/gvfs/third-party ]; then
+    FBCODE_BUILD="true"
+    # If we're compiling with TSAN we need pic build
+    PIC_BUILD=$COMPILE_WITH_TSAN
+    if [ -z "$ROCKSDB_FBCODE_BUILD_WITH_481" ]; then
+      source "$PWD/build_tools/fbcode_config.sh"
+    else
+      # we need this to build with MySQL. Don't use for other purposes.
+      source "$PWD/build_tools/fbcode_config4.8.1.sh"
+    fi
+fi
+
 # Delete existing output, if it exists
 rm -f "$OUTPUT"
 touch "$OUTPUT"