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.

3 files changed, 10 insertions, 0 deletions

diff --git a/Library/Homebrew/extend/ENV/shared.rb b/Library/Homebrew/extend/ENV/shared.rb
index c253942ac..c3bd9830a 100644
--- a/Library/Homebrew/extend/ENV/shared.rb
+++ b/Library/Homebrew/extend/ENV/shared.rb
@@ -293,6 +293,8 @@ module SharedEnvExtension
 
   def permit_arch_flags; end
 
+  def permit_weak_imports; end
+
   private
 
   def cc=(val)
diff --git a/Library/Homebrew/extend/os/mac/extend/ENV/std.rb b/Library/Homebrew/extend/os/mac/extend/ENV/std.rb
index d9cabc50e..21c375a76 100644
--- a/Library/Homebrew/extend/os/mac/extend/ENV/std.rb
+++ b/Library/Homebrew/extend/os/mac/extend/ENV/std.rb
@@ -146,4 +146,8 @@ module Stdenv
 
     append "CFLAGS", "-I#{MacOS::X11.include}" unless MacOS::CLT.installed?
   end
+
+  def permit_weak_imports
+    remove "LDFLAGS", "-Wl,-no_weak_imports"
+  end
 end
diff --git a/Library/Homebrew/extend/os/mac/extend/ENV/super.rb b/Library/Homebrew/extend/os/mac/extend/ENV/super.rb
index e482020e8..188008b1f 100644
--- a/Library/Homebrew/extend/os/mac/extend/ENV/super.rb
+++ b/Library/Homebrew/extend/os/mac/extend/ENV/super.rb
@@ -107,6 +107,10 @@ module Superenv
     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