tools/flex: respect STAGING_DIR_HOST

flex currently leaks the path of m4 as found on the build host.
While it is possible to override this using the M4 environment
variable (which we always did for autotools based builds) when
using CMake or Ninja the M4 variable is not set.
One easy fix is to make flex take STAGING_DIR_HOST into account
and expect m4 there if that variable is set in the environment.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>