AM_PATH_GLIB_2_0 m4 macro actually computes this value using
$PKG_CONFIG. Yet $PKG_CONFIG variable is the pkg-config tool looking for
target libraries (not host), hence it would return the executable
`glib-compile-resources` built for the target.

Also using the same variable name invalidates our test: our own
AC_PATH_PROG was never run as the variable was already set. And no
environment variable could override this test anymore either. This is
why I rename the test variable to HOST_GLIB_COMPILE_RESOURCES.

(cherry picked from commit d1d9eb17)