build: Move AM_CFLAGS and AM_CPPFLAGS to the top of Makefile.am

AM_CFLAGS and AM_CPPFLAGS aren't positional, so putting them at a
random place in Makefile.am can be misleading.

Signed-off-by: Derek Foreman <derekf@osg.samsung.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>