Resolve meson minimum version and deprecation warnings

Merged Dan Yeaw requested to merge danyeaw/pygobject:meson-update into master

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()

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