Stop copying makefiles around in build_extensions.sh

[cmsj]

There must be a way to use make -f with the generic Makefile, and still be able to infer the name of the module within the Makefile itself. Right now it doesn't work because of the first two lines of the generic Makefile.

[cmsj]

I'm circling back to the extension stuff at the moment, mostly because I'm on an underpowered Mac for a few weeks, and building all of the extensions every time I build Hammerspoon, is driving me crazy.

So, I'm (for probably the 4th or 5th time) attacking the problem of how to get Xcode to build the extensions natively, instead of using scripts and Makefiles.

I think we need to make this change eventually, and I'm happy to do the work now, but it is going to need some changes in the structure of our app bundle. It doesn't have to mean any changes for third party modules though, so let's not get worried about that.

Given the current experiment I have going, I believe I can get the extensions to build as dylibs (really just a .so) and be copied into the app bundle, but I'm not sure I can easily preserve the structure of Resources/extensions/hs/

Right now it's looking like we'll continue to have Resources/extensions/hs/foo/init.lua, but then the C part will be Resources/extensions/hs/libfoo.dylib, which means the init.lua would need to have a different require() for its companion C library. This is a slightly wrinkle for anyone who's doing external builds of an extension we ship, but I'm not sure if anyone is actually doing that ( @asmagill ?)

I'll keep banging at it and see what I can come up with, but ultimately I think I would prefer to disturb the third party workflow, to get our build system under control.