meson: add option to exclude gtk2 gtk2-module
Trying to build at-spi2-core statically in a 100% static system fails because of the shared library in atk-adaptor/gtk-2.0.
According to the README of at-spi2-atk the library should only be needed by GTK2 applications.
Add a meson option to exclude the shared library if one can ignore GTK2 support.