libgcrypt: add temporary Sierra build fix

Formula/libgcrypt.rb

@@ -25,6 +25,12 @@ class Libgcrypt < Formula
 
   def install
     ENV.universal_binary if build.universal?
+    # Temporary hack to get libgcrypt building on macOS 10.12.
+    # Seems to be a Clang issue rather than an upstream one, so
+    # keep checking whether or not this is necessary.
+    # Should be reported to GnuPG if still an issue when near stable.
+    # https://github.com/Homebrew/homebrew-core/issues/1957
+    ENV.O1 if MacOS.version >= :sierra
 
     system "./configure", "--disable-dependency-tracking",
                           "--disable-silent-rules",