modules/python: add some more options around path and config_vars
modules/python: add some more options around path and config_vars

What is actually defined here varies wildly on different python-versions
for different platforms.
On my python2.7 on Windows len(sysconfig.get_config_vars()) returns 17,
whereas in my Ubuntu that number is 517!

Hence it is useful to be able to check which keys are available, as
well as allowing specifying a default option.

thiblahute/meson,QuLogic/meson,becm/meson,jpakkane/meson,pexip/meson,QuLogic/meson,thiblahute/meson,becm/meson,thiblahute/meson,pexip/meson,thiblahute/meson,thiblahute/meson,jpakkane/meson,jeandet/meson,MathieuDuponchelle/meson,pexip/meson,jeandet/meson,becm/meson,jeandet/meson,QuLogic/meson,becm/meson,MathieuDuponchelle/meson,pexip/meson,jeandet/meson,becm/meson,MathieuDuponchelle/meson,jpakkane/meson,becm/meson,jpakkane/meson,jpakkane/meson,pexip/meson,jeandet/meson,jpakkane/meson,pexip/meson,QuLogic/meson,mesonbuild/meson,mesonbuild/meson,QuLogic/meson,mesonbuild/meson,thiblahute/meson,jeandet/meson,jeandet/meson,mesonbuild/meson,becm/meson,becm/meson,thiblahute/meson,jeandet/meson,jpakkane/meson,jpakkane/meson,becm/meson,thiblahute/meson,MathieuDuponchelle/meson,QuLogic/meson,pexip/meson,MathieuDuponchelle/meson,MathieuDuponchelle/meson,MathieuDuponchelle/meson,mesonbuild/meson,mesonbuild/meson,MathieuDuponchelle/meson,QuLogic/meson,mesonbuild/meson,jeandet/meson,thiblahute/meson,pexip/meson,mesonbuild/meson,jpakkane/meson,mesonbuild/meson,MathieuDuponchelle/meson,mesonbuild/meson,QuLogic/meson,pexip/meson,pexip/meson,QuLogic/meson,becm/meson