mk: fix gcc-5 version on Suse

In openSUSE Tumbleweed (and in any other SUSE distribution which
uses (or will use) gcc >= 5), gcc -dumpversion returns '5'. This is on
purpose as discussed in https://bugzilla.opensuse.org/show_bug.cgi?id=941428
As a result of which, the gcc-4.x comparison (40 against 5) does not
work leading to tons of warnings and failures during build.

This patch aims to change the way the gcc version is obtained by using
the gcc macros directly.
This is similar to what's being used in the Linux kernel. Querying the
GCC macros directly gives more accurate results compared to -dumpversion
which could vary across distributions.

Signed-off-by: Markos Chandras <mchandras@suse.de>
Acked-by: Thomas Monjalon <thomas.monjalon@6wind.com>