make CentOS3/RHEL3 detection work on RHEL3

see #4 (comment)

fixes issue 4

setup.py

@@ -6,7 +6,7 @@
 # https://bitbucket.org/ambroff/greenlet/issue/11/segfault-on-openbsd-i386
 # https://github.com/python-greenlet/greenlet/issues/4
 if ((sys.platform == "openbsd4" and os.uname()[-1] == "i386")
-    or ".EL-i686-with-redhat-3." in platform.platform()):
+    or ("-with-redhat-3." in platform.platform() and platform.machine() == 'i686')):
     os.environ["CFLAGS"] = ("%s %s" % (os.environ.get("CFLAGS", ""), "-Os")).lstrip()
 
 try: