Autotools: Always run glib-genmarshal, even if cross-compiling
glib-genmarshal is now written in Python rather than C, so we can always run it directly, even if we're cross-compiling for an architecture whose binaries we can't run. This matches the behaviour of the Meson build system.
This appears to fix Autotools cross-compilation in Debian, which regressed with 2.58.0.
Fixes #1546 (closed)