Allow setting the path to the Vala compiler using the VALAC env variable

[astavale]

It looks as though the executable for valac (and other compilers) is hard coded in /mesonbuild/environment.py :

def detect_vala_compiler(self):
    exelist = ['valac']

When developing the Vala compiler it is useful to build programs against a newly patched version of valac. With C the HowToX suggests the CC variable can be used:

Set compiler
When first running Meson, set it in an environment variable.
$ CC=mycc meson <options>

Is this the best way? So for Vala it would be:
$ VALAC=/home/development-vala/installed/bin/valac meson <options>

Another, maybe clearer, approach is to treat this as a cross compilation. So a cross file is created with the relevant binaries:

[binaries]
valac = '/home/development-vala/installed/bin/valac'

Then this is compiled with:
meson build --cross-file development_vala.txt

Thoughts?

[nirbheek]

We should definitely read VALAC. I thought we already did that, but I just checked and we don't.

Treating the run as a cross-compilation is wrong though, because it has several non-obvious effects (such as, tests won't be run without an exe_wrapper, built programs won't be run, etc). However, we were discussing a dependency file of sorts, so maybe it's good to extend that to also include this.

I'll rename this bug to be about VALAC specifically. In the meantime, I recommend symlinking that valac inside ~/.local/bin and adding that to your PATH.

[nirbheek]

Fixed by #1851