configure: Don't set user variables

CFLAGS is a user variable and shouldn't be set in either
configure.ac or Makefile.am because that makes it impossible to
override compiler flags at configure or make time.

AC_SUBST the default warning flags into AM_CFLAGS instead, which
is emitted before CFLAGS on the compiler command line.

Do the same thing with a mingw-specific flag as well.

Change-Id: I6ac3a3c8b7e0abf6f0e60ea239b549d68ea6d370
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/3870


Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>