compat: install the static import library

With libcompat installed (into $(prefix)/lib/vlc/ or whatever), libtool
will actually pull only needed functions from the static import library
libcompat.a. Previously, it would copy every compat functions into
every VLC binaries. This should shrink plugins a bit.

compat/Makefile.am

-noinst_LTLIBRARIES = libcompat.la
+pkglib_LTLIBRARIES = libcompat.la
 libcompat_la_SOURCES = dummy.c
 libcompat_la_LIBADD = $(LTLIBOBJS)
-libcompat_la_LDFLAGS = -no-undefined
+libcompat_la_LDFLAGS = -no-undefined -static
 
 BUILT_SOURCES = dummy.c
 CLEANFILES = dummy.c