Makefile.am: fix non-POSIX warning from automake

Makefile.am: fix non-POSIX warning from automake

Automake issues a warning
	Makefile.am:46: warning: wildcard $(srcdir: non-POSIX variable name
	Makefile.am:46: (probably a GNU make extension)
because the GNU make function 'wildcard' is not POSIX.
Also the GNU make function 'shell' triggers a similar warning.

Use the POSIX extension '!=', that executes an arbitrary shell
command, to replace the GNU make 'wildcard'.
Don't include the file 'NEWS' because automake already includes it
by default.

Change-Id: Ice560c3789cec4f3f2197a255d6f5af7b1fde634
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5965
Tested-by: jenkins
Reviewed-by: Jonathan McDowell <noodles-openocd@earth.li>