Remove constructor version attribute on OS X

Summary:
OS X doesn't support this -- it requires support at the linker
level which OS X just doesn't have. Reasonable OS X compilers will yell
about it, but even the ones that don't are just silently dropping it
on the floor.

So I'm not sure what //we// can do other than just silently dropping
this on the floor too and hoping that everything works.

Test Plan:
HHVM build on OS X gets ever so slightly further (runs into
other issues). HHVM build in fbcode still seems to build folly okay
(didn't wait for it to finish fully).

Fixes https://github.com/facebook/folly/issues/78

Reviewed By: tudorb@fb.com

FB internal diff: D1487191

Blame Revision: rFBCODE502be7cb0fda306ddc33b9c5ad7a53b0cfbdc893

folly/VersionCheck.h

@@ -67,6 +67,14 @@
  *
  * ... and then commpile your file with -DMYLIB_VERSION=\"1\"
  */
+
+#ifdef __APPLE__
+// OS X doesn't support constructor priorities. Just pray it works, I guess.
+#define FOLLY_VERSION_CHECK_PRIORITY __attribute__((constructor))
+#else
+#define FOLLY_VERSION_CHECK_PRIORITY __attribute__((constructor(101)))
+#endif
+
 // Note that this is carefully crafted: PRODUCT##Version must have external
 // linkage (so it collides among versions), versionCheck must have internal
 // linkage (so it does NOT collide between versions); if we're trying to have
@@ -75,7 +83,7 @@
 #define FOLLY_VERSION_CHECK(PRODUCT, VERSION) \
   const char* PRODUCT##Version = VERSION; \
   namespace { \
-  __attribute__((constructor(101))) void versionCheck() { \
+  FOLLY_VERSION_CHECK_PRIORITY void versionCheck() { \
     if (strcmp(PRODUCT##Version, VERSION)) { \
       fprintf(stderr, \
               "Invalid %s version: desired [%s], currently loaded [%s]\n", \