meson: use 0.50.0, default debugoptimized
meson: use 0.50.0, default debugoptimized

On NixOS this would always fail to build with meson 0.49.1
and with a release buildtype. I cannot reproduce the issue with >=0.50.0
and buildtype of debugoptimized. However release still brings the same
issues.

gnunn1/terminix,gnunn1/terminix,gnunn1/tilix,gnunn1/tilix,gnunn1/tilix