gettext (libintl-stub) was removed in commit [1], so the libintl-stub
lib and include directories aren't existing anymore. This commit cleans
up the INTL flags for the BUILD_NLS=n case.
[1] e6f569406f
Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>
Reviewed-by: Rosen Penev <rosenp@gmail.com>