allow spaces in $CC env

inspired from https://codereview.chromium.org/14813020/
--- xpra-0.15.7/setup.py.orig	2015-10-20 22:19:00.151484050 +0200
+++ xpra-0.15.7/setup.py	2015-10-20 22:23:40.268144880 +0200
@@ -426,7 +426,7 @@
 def get_gcc_version():
     global GCC_VERSION
     if len(GCC_VERSION)==0:
-        cmd = [os.environ.get("CC", "gcc"), "-v"]
+        cmd = os.environ.get("CC", "gcc").split() + ["-v"]
         r, _, err = get_status_output(cmd)
         if r==0:
             V_LINE = "gcc version "