Fix installation with --disable-static

If --disable-static is passed to configure, then .a files won't be built
or installed. Make removing them in install-exec-hook not fatal if they
don't exist.

src/compositor/mutter/plugins/Makefile.am

@@ -16,7 +16,7 @@ pkglib_LTLIBRARIES = default.la
 # (There is no way to stop libtool generating static libs locally, and we
 # cannot do this globally because of libmetacity-private.so).
 install-exec-hook:
-	rm $(DESTDIR)$(pkglibdir)/*.a
-	rm $(DESTDIR)$(pkglibdir)/*.la
+	-rm $(DESTDIR)$(pkglibdir)/*.a
+	-rm $(DESTDIR)$(pkglibdir)/*.la
 
 endif