Stop Using Meson Deprecated Features

$ gtk-doc$ meson setup builddir
NOTICE: Future-deprecated features used:
 * 0.56.0: {'dependency.get_pkgconfig_variable'}
 * 0.64.0: {'copy arg in configure_file'}

• https://mesonbuild.com/Release-notes-for-0-64-0.html
• https://mesonbuild.com/Release-notes-for-0-56-0.html

Meson 0.64.0 was released in 2022.