mesonlib: fix meson detection (again)
mesonlib: fix meson detection (again)

Starting from 8fc424418720da4ef61bde9348f4cc1a149d1cb2, tests
failed on my system (python 3.6 arch) because
shutil.which('meson.py') returns 'meson.py', not './meson.py'.

Refactor that codepath by using os.path.isabs instead of
"m_dir == '.'", also remove the adjacent comment because
it doesn't make much sense.

QuLogic/meson,jpakkane/meson,MathieuDuponchelle/meson,thiblahute/meson,becm/meson,QuLogic/meson,QuLogic/meson,thiblahute/meson,jeandet/meson,mesonbuild/meson,MathieuDuponchelle/meson,mesonbuild/meson,MathieuDuponchelle/meson,mesonbuild/meson,pexip/meson,thiblahute/meson,jeandet/meson,becm/meson,jpakkane/meson,MathieuDuponchelle/meson,QuLogic/meson,MathieuDuponchelle/meson,pexip/meson,jpakkane/meson,QuLogic/meson,jeandet/meson,QuLogic/meson,mesonbuild/meson,QuLogic/meson,QuLogic/meson,mesonbuild/meson,becm/meson,jeandet/meson,jpakkane/meson,pexip/meson,jeandet/meson,mesonbuild/meson,jpakkane/meson,pexip/meson,becm/meson,pexip/meson,MathieuDuponchelle/meson,becm/meson,becm/meson,mesonbuild/meson,jpakkane/meson,thiblahute/meson,mesonbuild/meson,mesonbuild/meson,jpakkane/meson,becm/meson,becm/meson,MathieuDuponchelle/meson,pexip/meson,pexip/meson,QuLogic/meson,thiblahute/meson,pexip/meson,MathieuDuponchelle/meson,jeandet/meson,thiblahute/meson,jeandet/meson,becm/meson,jeandet/meson,jeandet/meson,MathieuDuponchelle/meson,pexip/meson,thiblahute/meson,jpakkane/meson,mesonbuild/meson,thiblahute/meson,pexip/meson,thiblahute/meson,jpakkane/meson,becm/meson