Makefile: Fix bad system header flag

gcc does not guarantee support for -isystem=<directory> syntax. It
typically expects the flag to be in the form of 2 arguments `-isystem
<directory>`. Update this so it works with all versions of gcc.

Change-Id: I324446b414bc327961641c39dd36a3131c3bc5e1
Signed-off-by: William A. Kennington III <wak@google.com>
1 file changed