build: Let install-lib-links.mk handle .la files in subdirectories.

build: Let install-lib-links.mk handle .la files in subdirectories.

The next patches are going to combine some of the mapi subdirectories'
Makefiles into a single Makefile, giving better build parallelism.

lib_LTLIBRARIES will be set to something like

   lib_LTLIBRARIES = shared-glapi/libglapi.la es2api/libGLESv2.la

and the current code in install-lib-links.mk simply prepends .libs/ and
replaces the .la in order to create the filenames that it needs to ln/cp
into the LIBDIR. This doesn't work when the .la file is actually in a
subdirectory.

This patch fixes this and puts .libs/ in the right place.

Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>