Support system-wide installations of include files

[TC01]

As per a recent conversation on IRC with @alberthdev; it'd be really nice if we could install the include files (contents of inc/) somewhere and spasm-ng would automatically pick up on them, at least on Linux.

Some logs:

<TC01> is there somewhere they can be installed systemwide that spasm will just pick up on?
<TC01> (I was under the impression there was not)
<alberth> nope, not atm
<alberth> that's something I was kinda thinking about actually
<alberth> on windows (maybe mac), do executable path (e.g. ~/spasm-ng/spasm is the exec, so automatically include ~/spasm-ng/inc), and then maybe on linux do /usr/include/spasm-ng (or /usr/share/spasm-ng/inc?)
<alberth> if you raise an issue with that, I'll put it on my todo list :D
<TC01> usr/share might make more sense here, since these are for cross-compilation-- I'm not sure
<TC01> okay :)
-*- TC01 -> github issues
<alberth> yeah I was wondering, since /usr/include tends to smell like C rather than Z80 asm...
<TC01> The FHS unfortunately doesn't say where cross-compiler stuff should be installed... Fedora says that /usr/TARGET (where TARGET is, well, your target) is an acceptable place to put stuff but I don't know if anyone else agrees
<TC01> share might be the simplest place

[tari]

I would prefer to treat any packaged includes as specific to the assembler, since there is no real common subset of Z80 that is supported by all (or even most) tools. Thus my vote would be for something like $PREFIX/lib/spasm-ng by default, but also allowing for choice of any directory at compile-time.