Make configure better at finding Lua

[nielsm]

We've known for a long time that various systems put Lua in different locations with different header names and different library names. However that shouldn't be an excuse for giving everyone wanting to build Aegisub a hard time by forcing them to look up their system's location and pass variables to configure, it's very hard to discover that that's what you need to do and seems to be one of the biggest showstoppers for other people building Aegisub.

I believe that at least some Linux distributions may be writing their own .pc files for Lua, and in that case we should try to use those, and otherwise keep a list of common locations around. It seems lame, but if that's what it takes to make compilation smoother then we should do it. (Alternatively, always build with our in-tree Lua.)

[verm]

I'll see what I can do I've thought about this before but I really didn't want to open this can of worms. Once you make things easier for users to find in one area then they expect it in others. Ultimately once we release a unix version most distributions will have a port/package for it so there won't be much end-user compiling.

Also, of the reports I've gotten finding LUA is the least reported issue. FFMPEG is the one that causes 90% or more of the headaches.

I'll take this and see what I can do, if anything..

[Kovensky]

Debian installs lua as liblua5.1.so, so you should check for -llua5.1

The headers are installed on /usr/include/lua5.1/

[verm]

Needs to be done for milestone:2.1.8

[verm]

(In [3801]) Add two new macros (custom) to search for headers in different directories and libraries with different names. updates #989.

[verm]

(In [3802]) Use the two new macros to supply auto-detection for lua. updates #989.

[verm]

This is done, I forgot to reference this ticket in r3823 and r3824 which merge changes from r3801 and r3802.