meson: fix warning for using too-new features

meson: fix warning for using too-new features

In commit 031de3c69ccbf3282ed02fb49369b476730aeca8 a feature of Meson
0.50.0 was added, but the minimum specified version of Meson is 0.48.0.
Meson therefore emitted a warning:

WARNING: Project targets '>=0.48.0' but uses feature introduced in '0.50.0': required arg in compiler.has_header.

And if anyone actually used Meson 0.48.0 to build with, it would error
out with mysterious claims that the build file itself is invalid, rather
than telling the user to install a newer version of Meson.

Solve this by bumping the minimum version to align with reality. This
e.g. drops support for Debian oldstable (buster)'s packaged version of
Meson, but still works if backports are enabled, or if the user can
`pip install` a newer version.