Resolve meson minimum version and deprecation warnings

A minimum version of 0.60.0 was chosen to match the version used by GTK.

1. get_pkgconfig_variable('variable') is replaced with get_variable(pkgconfig : 'variable')
2. build_root is replaced by project_build_root(), unless used by a subproject, and then we would need to use global_build_root() or current_build_dir()