Makefile: Ensure we build with -std=gnu11

As many targets are now commonly built with gcc-6 or later (which
defaults to a newer C standard than older compilers), certain C
constructs are now being used as they produce more readable code.  And
while all compilers that we support building with support the C11
standard (and GNU11) they do not default to that standard.  Ensure that
we pass along -std=gnu11 when building.

Signed-off-by: Tom Rini <trini@konsulko.com>