OL_MKDEPEND: Round 2

now supports DETECTION using:
	./configure
	env MKDEP_FLAGS=-MM ./configure
	env MKDEP=foo ./configure (no tests, no flags)
	env MKDEP=foo MKDEP_FLAGS=-bar ./configure (no tests)
Exported AC_MKDEP and AC_MKDEP_FLAGS which can no be used
in build/*.mk