Simplify Makefile: Don't query for default LUA_PATH

All Lua versions since at least Lua 5.1 replace ";;" in $LUA_PATH and $LUA_CPATH
with the default value. Use this instead of trying to query the default path so
that we can prepend something.

If $LUA_PATH is empty, we end up with ";;" in the new value, so the default path
is added. If $LUA_PATH is not empty, we add our own entry to it.

This also fixes a (theoretical) issue due to missing quoting on the path.

Signed-off-by: Uli Schlachter <psychon@znc.in>

tests/Makefile

@@ -57,8 +57,8 @@ clean :
 check : all
 	cd .. && LD_LIBRARY_PATH=tests:$$LD_LIBRARY_PATH \
 	    GI_TYPELIB_PATH=tests:$$GI_TYPELIB_PATH \
-	    LUA_PATH=./?.lua\;`$(LUA) -e "print(package.path)"` \
-	    LUA_CPATH=./?.so\;`$(LUA) -e "print(package.cpath)"` \
+	    LUA_PATH="./?.lua;${LUA_PATH};" \
+	    LUA_CPATH="./?.so;${LUA_CPATH};" \
 	    $(LUA) tests/test.lua
 
 $(REGRESS) : regress.o