Link to FB internal build of ZSTD with -fPIC (#4249)

Summary:
TSAN requires the code is built with -fPIC. This PR links against a libzstd built with -fPIC when necessary, which enables ZSTD compression to be used in TSAN builds.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4249

Differential Revision: D9244746

Pulled By: ajkr

fbshipit-source-id: 8c6a8fadd6c8643b2077afcbc3626779e1d73b63

build_tools/fbcode_config.sh

@@ -43,11 +43,15 @@ if test -z $PIC_BUILD; then
   LZ4_INCLUDE=" -I $LZ4_BASE/include/"
   LZ4_LIBS=" $LZ4_BASE/lib/liblz4.a"
   CFLAGS+=" -DLZ4"
+fi
 
-  ZSTD_INCLUDE=" -I $ZSTD_BASE/include/"
+ZSTD_INCLUDE=" -I $ZSTD_BASE/include/"
+if test -z $PIC_BUILD; then
   ZSTD_LIBS=" $ZSTD_BASE/lib/libzstd.a"
-  CFLAGS+=" -DZSTD"
+else
+  ZSTD_LIBS=" $ZSTD_BASE/lib/libzstd_pic.a"
 fi
+CFLAGS+=" -DZSTD"
 
 # location of gflags headers and libraries
 GFLAGS_INCLUDE=" -I $GFLAGS_BASE/include/"