meson: use python3 module to find python3

This handy helper is nice for OSes that are not linux or BSD like (mac
and windows) as it knows how to find python3 in odd places.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

meson.build

@@ -697,7 +697,7 @@ if with_platform_haiku
   pre_args += '-DHAVE_HAIKU_PLATFORM'
 endif
 
-prog_python = find_program('python3')
+prog_python = import('python3').find_python()
 has_mako = run_command(
   prog_python, '-c',
   '''