gallium/osmesa: Link, not copy, the shared library to the LIB_DIR.

Just like all other mesa libraries...

CC: "9.2" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/gallium/targets/osmesa/Makefile.am

@@ -75,7 +75,7 @@ all-local: lib@OSMESA_LIB@.la
 	$(MKDIR_P) $(top_builddir)/$(LIB_DIR)/gallium;
 	ln -f .libs/lib@OSMESA_LIB@.so $(top_builddir)/$(LIB_DIR)/gallium/lib@OSMESA_LIB@.so;
 	ln -f .libs/lib@OSMESA_LIB@.so.@OSMESA_VERSION@ $(top_builddir)/$(LIB_DIR)/gallium/lib@OSMESA_LIB@.so.@OSMESA_VERSION@;
-	cp .libs/lib@OSMESA_LIB@.so.@OSMESA_VERSION@.0.0 $(top_builddir)/$(LIB_DIR)/gallium/
+	ln -f .libs/lib@OSMESA_LIB@.so.@OSMESA_VERSION@.0.0 $(top_builddir)/$(LIB_DIR)/gallium/
 endif
 
 pkgconfigdir = $(libdir)/pkgconfig