Make tup compilation more similar to make in regard to selected files

If ASSOURCES, CSOURCES or CXXSOURCES is defined, then use these as the
list of files to compile. Otherwise search for all files with proper
extension.

Note that these variables should be tables, not strings.

compile.lua

@@ -1,20 +1,20 @@
 --
 -- file: compile.lua
 --
--- author: Copyright (C) 2014-2015 Kamil Szczygiel http://www.distortec.com http://www.freddiechopin.info
+-- author: Copyright (C) 2014-2016 Kamil Szczygiel http://www.distortec.com http://www.freddiechopin.info
 --
 -- This Source Code Form is subject to the terms of the Mozilla Public License, v. 2.0. If a copy of the MPL was not
 -- distributed with this file, You can obtain one at http://mozilla.org/MPL/2.0/.
 --
 
-for index, filename in ipairs(tup.glob("*.S")) do
+for index, filename in ipairs(ASSOURCES or tup.glob("*.S")) do
 	as(filename)
 end
 
-for index, filename in ipairs(tup.glob("*.c")) do
+for index, filename in ipairs(CSOURCES or tup.glob("*.c")) do
 	cc(filename)
 end
 
-for index, filename in ipairs(tup.glob("*.cpp")) do
+for index, filename in ipairs(CXXSOURCES or tup.glob("*.cpp")) do
 	cxx(filename)
 end