Changes default macosx compiler to clang.

The default gcc version provided by current Xcode versions doesn't work.
Alternatively macport's gcc-4.7 is known to work too, but that is less
common for macosx users.

config/nimrod.cfg

@@ -91,6 +91,7 @@ hint[LineTooLong]=off
 @end
 
 @if macosx:
+  cc = clang
   tlsEmulation:on
   gcc.options.always = "-w -fasm-blocks"
   gpp.options.always = "-w -fasm-blocks"