superenv: add LDFLAGS if disabling weak imports.

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

@@ -81,6 +81,10 @@ module Superenv
     s << "s" if MacOS.version >= :mountain_lion
     # Fix issue with >= 10.8 apr-1-config having broken paths
     s << "a" if MacOS.version >= :mountain_lion
+    # Xcode 8 should be told to fail to link against weak links
+    # Issue from Apple engineer:
+    # https://github.com/Homebrew/homebrew-core/issues/3727
+    s << "w" if no_weak_imports?
     s
   end
 

Library/Homebrew/shims/super/cc

@@ -258,8 +258,10 @@ class Cmd
     case mode
     when :ld
       args << "-headerpad_max_install_names"
+      args << "-no_weak_imports" if no_weak_imports?
     when :ccld, :cxxld
       args << "-Wl,-headerpad_max_install_names"
+      args << "-Wl,-no_weak_imports" if no_weak_imports?
     end
     args
   end
@@ -305,6 +307,10 @@ class Cmd
     config.include?("K")
   end
 
+  def no_weak_imports?
+    config.include?("w")
+  end
+
   def canonical_path(path)
     path = Pathname.new(path)
     path = path.realpath if path.exist?