Meson: Cross file does not allow passing Vala compiler and args

[oleavr]

Which makes sense based on the current logic:

        elif lang == 'vala':
            comp = self.environment.detect_vala_compiler()
            if need_cross_compiler:
                cross_comp = comp  # Vala compiles to platform-independent C

But I would argue that it is very useful to be able to pass -DFOO and
do #if FOO in Vala code, for the few cases where platform-dependent
Vala code is needed.

[oleavr]

Working on a PR addressing this, I'll submit it later today.