Makefile: Explicitly set '-std=c99'

This matches the build flags that are correctly explicitly defined in
meson.build where-as the Makefile is randomly picking up whatever the
system toolchain happens to default to.

Fix dmi.c while we are here to avoid a re-define of _GNU_SOURCE.

BUG=none
TEST=`make` with both gcc and clang.

Change-Id: I4f973927fc018510a3beaa6c4fa2f356c77c7a6e
Signed-off-by: Edward O'Callaghan <quasisec@google.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/47908
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>
Reviewed-by: Sam McNally <sammc@google.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
2 files changed