1. Use cross-platform `/` operator for path construction.
2. Use `meson.project_source_root()` for correct path resolution of
generate_test_runner.rb path when used as a subproject.
3. Bump the minimum required Meson version to '0.56.0' as this is
needed for the above changes.
The main project doesn't really have any specific version requirement.
Specify a very low one just in case -- 0.37.0 is old enough to cover
probably any existing use of Meson anywhere in the wild, and
coincidentally is also the version that Meson started adding feature
warnings for, to notify you if you use too-new features.
The example *does* depend on a specific version. It needs 0.55.0 in
order to use subproject wrap dependency fallback instead of the legacy
style of specifying the name of the variable as a fallback. Ensure that
is used.
Cleaned up compiler flags in root meson.build, renamed a variable and removed 'c_std=<standard version>' because Meson sets C standard flag to C11 by default.
