build: Avoid unnecessary format() when building long options

When building a custom_target command line,

'--opt=@0@'.format(x.full_path())

can be replaced with

'--opt', x

which is simpler and avoids hiding the x dependency from Meson. In some cases we were declaring that dependency explicitly; stop doing so.

This should fix an occasional build error from build-aux/gen-resources.py when glib is built as a parallel subproject:

FileNotFoundError: [Errno 2] No such file or directory: '[...]/glib-2.80.0/gio/glib-compile-resources'

We can't just add glib_compile_resources to depends because depends doesn't accept external_program arguments.