gallium/targets: don't leave an empty target directory(ies)

gallium/targets: don't leave an empty target directory(ies)

Some drivers do not support certain targets - for example nouveau
doesn't do VAAPI, while freedreno doesn't do of the video backends.

As such if we enter vdpau when building freedreno/ilo/etc, a vdpau/
folder will be created, empty library will be build and almost
immediately removed. Thus keeping an empty vdpau/ folder around.

There are two ways to fix this.

 * add substantial tracking in configure/makefiles so that we never end
up in targets/vdpau
 Downsides:
Error prone, as the configure checks and the 'include
gallium/drivers/foo/Automake.inc' can easily get out of sync.

 * remove the folder, if empty, alongside the empty library.
 Downsides:
In the latter case vdpau/ might be empty before the mesa build has
started, yet we'll remove it either way.

This patch implements the latter option, as the downside isn't that
significant, plus the patch is way shorter ;-)

v2: use has_drivers to track since TARGET_DRIVERS can contain space,
hence neither string comparison nor -n/-z works correctly.

Gentoo Bugzilla: https://bugs.gentoo.org/545230
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>