We don't need to run binaries we just built in order to successfully
build GLib and friends any more.
Since commit b74e2a7, we don't need to run glib-genmarshal when building
GIO; since commit f9eb9eed, all our tests (including the ones that do
need to run binaries we just built) are only built when running "make
check", instead of unconditionally at every build.
This means that we don't need to check for existing, native binaries
when cross-compiling, and fail the configuration step if they are not
found — which also means that you don't need to natively build GLib for
your toolchain, in order to cross-compile GLib.
We can also use the cross-compilation conditional, and skip those tests
that require a binary we just built in order to build.
https://bugzilla.gnome.org/show_bug.cgi?id=753745
5ce70917df