strklos: use external bdw-gc

This adds 'bdw-gc' as a dependency and fixes compilation on systems
where ucontext is deprecated. We could just duplicate the fix here, but
listing it as a dependency instead also avoids a symlink race for the
bdw-gc libraries.

Should fix Homebrew/homebrew#7423, for real this time.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/stklos.rb

@@ -7,6 +7,7 @@ class Stklos < Formula
 
   depends_on 'gmp'
   depends_on 'pcre'
+  depends_on 'bdw-gc'
 
   # it fails with LLVM while building its internal bdw-gc,
   # for the same reason as bdw-gc itself.