Fix GCC on Darwin for Python 3.

distutils.sysconfig.get_config_var('CC') returns the compiler used to
build Python, which may be not installed on the system where uWSGI is
being built.

In my case, I grabbed Python 3.3.2 from python.org: it was apparently
compiled with 'gcc-4.2', but my system only has 'gcc' (symlink) and
'llvm-gcc-4.2'.  This change tries to detect this situation (only on
Darwin) and sets GCC to 'llvm-' + compiler returned by sysconfig.

uwsgiconfig.py

@@ -476,6 +476,8 @@ def open_profile(filename):
 class uConf(object):
 
     def __init__(self, filename, mute=False):
+        global GCC
+
         self.config = ConfigParser.ConfigParser()
         if not mute:
             print("using profile: %s" % filename)
@@ -528,11 +530,10 @@ class uConf(object):
         if uwsgi_os == 'GNU':
             self.cflags.append('-D__HURD__')
 
-        try:
-            gcc_version = str(spcall("%s -dumpversion" % GCC))
-        except:
-            print("*** you need a c compiler to build uWSGI ***")
-            sys.exit(1)
+        gcc_version = spcall("%s -dumpversion" % GCC)
+        if not gcc_version and GCC.startswith('gcc') and uwsgi_os == 'Darwin':
+            GCC = 'llvm-' + GCC
+            gcc_version = spcall("%s -dumpversion" % GCC)
 
         try:
             add_it = False