Don't reject -L/usr/local/lib when SDKROOT is defined

/usr/local/lib is removed from the default linker search path when
SDKROOT is defined or sysroot is specified.

Homebrew sometimes sets SDKROOT without setting a sysroot as of 45e138f.

Library/Homebrew/shims/super/cc

@@ -279,7 +279,9 @@ class Cmd
   end
 
   def system_library_paths
-    %W[#{sysroot}/usr/lib /usr/local/lib]
+    paths = ["#{sysroot}/usr/lib"]
+    paths << "/usr/local/lib" unless sysroot || ENV["SDKROOT"]
+    paths
   end
 
   def configure?