Move -fPIC into YOGA_DEFS

Summary:
Move -fPIC from BUCK to YOGA_DEFS, so that it can be overridden when
building yoga inside another build tree (since the larger build tree
may need its own configuration for -fPIC). No change when building yoga
standalone.

Reviewed By: emilsjolander

Differential Revision: D4714782

fbshipit-source-id: c706336cda72b36045e744e4fcaea4c0899bcf38

BUCK

@@ -12,9 +12,8 @@ GMOCK_OVERRIDE_FLAGS = [
     "-Wno-inconsistent-missing-override",
 ]
 
-COMPILER_FLAGS = BASE_COMPILER_FLAGS + [
+COMPILER_FLAGS = LIBRARY_COMPILER_FLAGS + [
     "-std=c11",
-    "-fPIC",
 ]
 
 TEST_COMPILER_FLAGS = BASE_COMPILER_FLAGS + GMOCK_OVERRIDE_FLAGS + ["-std=c++11"]

YOGA_DEFS

@@ -33,6 +33,10 @@ BASE_COMPILER_FLAGS = [
   '-O3',
 ]
 
+LIBRARY_COMPILER_FLAGS = BASE_COMPILER_FLAGS + [
+  '-fPIC',
+]
+
 def yoga_dep(dep):
   return '//' + dep