exists, libtool will add all libraries libB.la refers to (dependency_libs
field) to the linker command line and store them in the dependency_libs
field of libA.la.  So everything that subsequently links with libA will also
link to these extra libraries.  This causes too much overlinking.

This commit modifies Mk/Uses/libtool.mk so it empties the dependency_libs
field in .la libraries during staging.  However, because .la libraries have
very limited use when dependency_libs is empty it makes sense to completely
remove them during staging.

So with this commit USES=libtool is modified to remove .la libraries and a
new form (USES=libtool:keepla) is introduced in case they need to be kept
(dependency_libs is still emptied).

PORTREVISION is bumped on all ports with USES=libtool that install .la
libraries.  Most ports are also changed to add :keepla because .la
libraries have to be kept around as long as there are dependent ports with
.la libraries that refer to them in their dependency_libs field.  In most
cases :keepla can be removed again as soon as all dependent ports that
install .la libraries have some form of USES=libtool added to their
Makefile.

PR:		ports/188759
Exp-run:	bdrewery
Approved by:	portmgr (bdrewery)