Cygwin build not to use -fPIC

Summary:
Cygwin doesn't support -fPIC. Remove it.
Not sure whether we can build shared library in Cygwin but at least it can build without warning.

Test Plan: Build under Cygwin

Reviewers: yhchiang, rven, kradhakrishnan, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D40077

build_tools/build_detect_platform

@@ -157,6 +157,7 @@ case "$TARGET_OS" in
         ;;
     Cygwin)
         PLATFORM=CYGWIN
+        PLATFORM_SHARED_CFLAGS=""
         PLATFORM_CXXFLAGS="-std=gnu++11"
         COMMON_FLAGS="$COMMON_FLAGS -DCYGWIN"
         if [ -z "$USE_CLANG" ]; then