makefile: default jsregexp-CC to system-CC.

this is for systems where gcc may not be available/called something
else. We assume that $CC is set to something valid, or that cc is
available (since that's the fallback for $CC in gnu-make if $CC is not
set).

Makefile

@@ -46,8 +46,8 @@ JSREGEXP005_PATH=deps/jsregexp005
 jsregexp:
 	git submodule init
 	git submodule update
-	$(MAKE) "INCLUDE_DIR=-I'$(shell pwd)/deps/lua51_include/'" LDLIBS="${LUA_LDLIBS}" -C "${JSREGEXP_PATH}"
-	$(MAKE) "INCLUDE_DIR=-I'$(shell pwd)/deps/lua51_include/'" LDLIBS="${LUA_LDLIBS}" -C "${JSREGEXP005_PATH}"
+	$(MAKE) "CC=$(CC)" "INCLUDE_DIR=-I'$(shell pwd)/deps/lua51_include/'" LDLIBS="${LUA_LDLIBS}" -C "${JSREGEXP_PATH}"
+	$(MAKE) "CC=$(CC)" "INCLUDE_DIR=-I'$(shell pwd)/deps/lua51_include/'" LDLIBS="${LUA_LDLIBS}" -C "${JSREGEXP005_PATH}"
 
 install_jsregexp: jsregexp
 	# remove old binary.