gettext-full: override SUBDIRS variable with Makefile

gettext-full: override SUBDIRS variable with Makefile

Instead of editing the SUBDIRS variable with a patch,
it can be overriden at the end of the command line when invoking Make.

This tool has a series of recursive Makefiles in each subdirectory,
therefore SUBDIRS is set to a pattern of Make functions
so that the result is variable depending on the current subdirectory
that Make is being invoked in.

Some of the subdirectories don't have a Makefile and are just storing files
for another subdirectory Makefile target,
therefore we have to place a fake Makefile that does nothing.

Signed-off-by: Michael Pratt <mcpratt@pm.me>