DragonflyBSD misc patches

The bug reporter anticipated a problem, instead of experiencing one.

IRC conversation:
<xtraeme> sem_init is defined in librt on NetBSD
	solaris uses libposix4 iirc
	that's why I sent a patch
<chadmiller> xtraeme: Agreed.  But, AFAICT, mysql doesn't use 
	sem_init() anywhere.  Thus my confusion.
<xtraeme> I didn't verify that, but I saw that sem_init wasn't detected 
	correctly...
	why are you checking for sem_init if it's unused then? :-)
<chadmiller> xtraeme: In recent autoconf scripts, we change it to test 
	for sched_yield() .
<xtraeme> that's ok then
	sched_yield is in libc
	feel free to close the bug report then :-)


configure.in:
  Remove useless test for sem_init() .