Add permit_weak_imports directive

Issue Homebrew/homebrew-core#3727 suggested we set -no_weak_imports for
new versions of Xcode to ensure that e.g. building on 10.11 against the
10.12 SDK doesn't result in a situation where autotools thinks symbols
exist that don't actually exist on the current platform.

Further discussion in golang/go#16770 revealed that some packages
require weak imports to build normally.

Library/Homebrew/extend/ENV/shared.rb

@@ -293,6 +293,8 @@ def warn_about_non_apple_gcc(name)
 
   def permit_arch_flags; end
 
+  def permit_weak_imports; end
+
   private
 
   def cc=(val)

Library/Homebrew/extend/os/mac/extend/ENV/std.rb

@@ -146,4 +146,8 @@ def x11
 
     append "CFLAGS", "-I#{MacOS::X11.include}" unless MacOS::CLT.installed?
   end
+
+  def permit_weak_imports
+    remove "LDFLAGS", "-Wl,-no_weak_imports"
+  end
 end

Library/Homebrew/extend/os/mac/extend/ENV/super.rb

@@ -107,6 +107,10 @@ def set_x11_env_if_installed
     ENV.x11 = MacOS::X11.installed?
   end
 
+  def permit_weak_imports
+    remove "HOMEBREW_CCCFG", "w"
+  end
+
   # These methods are no longer necessary under superenv, but are needed to
   # maintain an interface compatible with stdenv.
   alias_method :macosxsdk, :noop