Build universal binary for libffi (#258)

Author: straight-shoota

omnibus/config/software/libffi.rb

@@ -31,6 +31,8 @@
 
 build do
   env = with_standard_compiler_flags(with_embedded_path)
+  env["CFLAGS"] << " -fPIC -arch arm64 -arch x86_64"
+  env["CPPFLAGS"] = env["CPPFLAGS"].gsub("-arch arm64 -arch x86_64", "")
 
   env["INSTALL"] = "/opt/freeware/bin/install" if aix?