change the definition of FOLLY_MAYBE_UNUSED

Summary:
Currently gcc does not support attributes declared using square brackets well and some builds might break because of this.

Since `__attribute__((...))` works for both clang and gcc, we should switch to it for now.

Reviewed By: yfeldblum

Differential Revision: D5456734

fbshipit-source-id: d7a8bf1389afc5d60604f22d37c93611dc46259c

folly/CppAttributes.h

@@ -65,10 +65,8 @@
  *    #endif
  *  }
  */
-#if FOLLY_HAS_CPP_ATTRIBUTE(maybe_unused)
-#define FOLLY_MAYBE_UNUSED [[maybe_unused]]
-#elif FOLLY_HAS_CPP_ATTRIBUTE(gnu::unused)
-#define FOLLY_MAYBE_UNUSED [[gnu::unused]]
+#if FOLLY_HAS_CPP_ATTRIBUTE(__unused__)
+#define FOLLY_MAYBE_UNUSED __attribute__((__unused__))
 #else
 #define FOLLY_MAYBE_UNUSED
 #endif