environment: Read VALAC from the environment and use it if set

[alvarez86]

The valac binary was hard coded in meson. We now check if VALAC is
defined in the environment, and if it is, use its value as the vala
compiler, if not, we proceed with the hard coded binary name.

This is meant for #1645, I don't really know how to provide a test case for this, as it will
require two vala compilers in the environment. Nonetheless, I hope this will be useful,
even as a starting point.

[codecov-io]

Codecov Report

Merging #1851 into master will increase coverage by 7.98%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1851      +/-   ##
==========================================
+ Coverage   62.99%   70.98%   +7.98%     
==========================================
  Files          58       75      +17     
  Lines       16870    23261    +6391     
  Branches     3452     6290    +2838     
==========================================
+ Hits        10628    16511    +5883     
- Misses       5149     5573     +424     
- Partials     1093     1177      +84

Impacted Files	Coverage Δ
mesonbuild/environment.py	82.45% <100%> (+11.16%)	⬆️
mesonbuild/dependencies/misc.py	52.92% <0%> (-0.84%)	⬇️
mesonbuild/dependencies/__init__.py	100% <0%> (ø)	⬆️
mesonbuild/scripts/dist.py	0% <0%> (ø)	⬆️
mesonbuild/scripts/meson_install.py	0% <0%> (ø)	⬆️
mesonbuild/backend/xcodebackend.py	0% <0%> (ø)	⬆️
mesonbuild/modules/qt4.py	0% <0%> (ø)	⬆️
mesonbuild/scripts/vcstagger.py	0% <0%> (ø)	⬆️
mesonbuild/modules/rpm.py	0% <0%> (ø)	⬆️
mesonbuild/modules/modtest.py	100% <0%> (ø)	⬆️
... and 49 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update fe84a6f...9941632. Read the comment docs.

[oleavr]

This is also covered by #1808.

[jpakkane]

Since the issue will be fixed by #1808, let's close this.

[nirbheek]

Since #1808 is not going to be ready in time for the release, we should probably merge this for now. I think it's a small enough change that won't break anything so close to the release.

[nirbheek]

<al_t> yes, this is needed for getting vala itself built with meson. bootstrap and i think running the test suite as well

Looks like we have a pretty good reason for doing it now rather than later. :)