remove explicit target definitions for gmp, mpfr, and mpc.

Originally added in commit 40d6414f28,
the purpose of the above target definitions was to allow using the
static-only symlink variant of slibtool (i.e. slibtool-static) in
those build steps which required it. Given slibtool's newly added
ability to auto-detect its desired operation mode (shared-only,
static-only, or both), as well as the integration of rlibtool
support in mcm, the aforementioned explicit target definitions
are no longer needed.

litecross/Makefile

@@ -207,19 +207,7 @@ obj_gcc/.lc_configured: | obj_gcc src_gcc
 	cd obj_gcc && ../src_gcc/configure $(FULL_GCC_CONFIG)
 	touch $@
 
-obj_gcc/.lc_gmp_built: | obj_gcc/.lc_configured
+obj_gcc/gcc/.lc_built: | obj_gcc/.lc_configured
-	cd obj_gcc && $(MAKE) MAKE="$(MAKE) $(LIBTOOL_ARG)" all-gmp
-	touch $@
-
-obj_gcc/.lc_mpfr_built: | obj_gcc/.lc_gmp_built
-	cd obj_gcc && $(MAKE) MAKE="$(MAKE) $(LIBTOOL_ARG)" all-mpfr
-	touch $@
-
-obj_gcc/.lc_mpc_built: | obj_gcc/.lc_mpfr_built
-	cd obj_gcc && $(MAKE) MAKE="$(MAKE) $(LIBTOOL_ARG)" all-mpc
-	touch $@
-
-obj_gcc/gcc/.lc_built: | obj_gcc/.lc_mpc_built
 	cd obj_gcc && $(MAKE) MAKE="$(MAKE) $(LIBTOOL_ARG)" all-gcc
 	touch $@