restrict FOLLY_HAVE_WEAK_SYMBOLS

Summary: Restrict `FOLLY_HAVE_WEAK_SYMBOLS` by requiring not just that use of gnu weak symbols will compile but also that it will link and run when no definition is available for a weak declaration.

Reviewed By: luciang

Differential Revision: D25788660

fbshipit-source-id: 52447601257242cd713ac5cd70e900e3738a9770

CMake/FollyConfigChecks.cmake

@@ -128,10 +128,11 @@ check_cxx_source_compiles("
   }"
   FOLLY_HAVE_VLA
 )
-check_cxx_source_compiles("
-  extern \"C\" void configure_link_extern_weak_test() __attribute__((weak));
+check_cxx_source_runs("
+  extern \"C\" int folly_example_undefined_weak_symbol() __attribute__((weak));
   int main(int argc, char** argv) {
-    return configure_link_extern_weak_test == nullptr;
+    auto f = folly_example_undefined_weak_symbol; // null pointer
+    return f ? f() : 0; // must compile, link, and run with null pointer
   }"
   FOLLY_HAVE_WEAK_SYMBOLS
 )