Instead of using fragile guessing to figure out how to invoke meson,
set the value when meson is run. Also rework how we pass of
meson_script_launcher to regenchecker.py -- it wasn't even being used
With this change, we only need to guess the meson path when running
the tests, and in that case:
1. If MESON_EXE is set in the env, we know how to run meson
for project tests.
2. MESON_EXE is not set, which means we run the configure in-process
for project tests and need to guess what meson to run, so either
- meson.py is found next to run_tests.py, or
- meson, meson.py, or meson.exe is in PATH
Otherwise, you can invoke meson in the following ways:
1. meson is installed, and mesonbuild is available in PYTHONPATH:
- meson, meson.py, meson.exe from PATH
- python3 -m mesonbuild.mesonmain
- python3 /path/to/meson.py
- meson is a shell wrapper to meson.real
2. meson is not installed, and is run from git:
- Absolute path to meson.py
- Relative path to meson.py
- Symlink to meson.py
All these are tested in test_meson_commands.py, except meson.exe since
that involves building the meson msi and installing it.
Nirbheek Chauhan