fPIC in x64 environment

Summary:
Check https://github.com/facebook/rocksdb/pull/15 for context.

Apparently [1], we need -fPIC in x64 environments (this is added only in non-fbcode).

In fbcode, I removed -fPIC per @dhruba's suggestion, since it introduces perf regression. I'm not sure what would are the implications of doing that, but looks like it works, and when releasing to the third-party, we're disabling -fPIC either way [2].

Would love a suggestion from someone who knows more about this

[1] http://eli.thegreenplace.net/2011/11/11/position-independent-code-pic-in-shared-libraries-on-x64/
[2] https://our.intern.facebook.com/intern/wiki/index.php/Database/RocksDB/Third_Party

Test Plan: make check works

Reviewers: dhruba, emayanke, kailiu

Reviewed By: dhruba

CC: leveldb, dhruba, reconnect.grayhat

Differential Revision: https://reviews.facebook.net/D14337

build_tools/build_detect_platform

@@ -75,7 +75,7 @@ if test -z "$TARGET_OS"; then
     TARGET_OS=`uname -s`
 fi
 
-COMMON_FLAGS="$COMMON_FLAGS ${CFLAGS} -fPIC"
+COMMON_FLAGS="$COMMON_FLAGS ${CFLAGS}"
 CROSS_COMPILE=
 PLATFORM_CCFLAGS=
 PLATFORM_CXXFLAGS="$PLATFORM_CXXFLAGS ${CXXFLAGS}"
@@ -174,6 +174,12 @@ if [ "$CROSS_COMPILE" = "true" -o "$FBCODE_BUILD" = "true" ]; then
     # Also don't need any compilation tests if compiling on fbcode
     true
 else
+    # do fPIC on 64 bit in non-fbcode environment
+    case "$TARGET_OS" in
+        x86_64)
+            PLATFORM_CXXFLAGS="$PLATFORM_CXXFLAGS -fPIC"
+    esac
+
     # If -std=c++0x works, use <atomic>.  Otherwise use port_posix.h.
     $CXX $CFLAGS -std=c++0x -x c++ - -o /dev/null 2>/dev/null  <<EOF
       #include <atomic>