allow spaces in $CC env

inspired from https://codereview.chromium.org/14813020/
--- xpra-0.10.10/setup.py~	2013-12-19 11:14:12.000000000 +0200
+++ xpra-0.10.10/setup.py	2013-12-30 20:14:28.220987556 +0200
@@ -284,7 +284,8 @@
 def get_gcc_version():
     global GCC_VERSION
     if len(GCC_VERSION)==0:
-        cmd = [os.environ.get("CC", "gcc"), "-v"]
+        cc = os.environ.get('CC', 'gcc').split()
+        cmd = cc + ['-v']
         proc = subprocess.Popen(cmd, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
         output, _ = proc.communicate()
         status = proc.wait()