disable DV-support if libiec61883 or libraw1394 are missing

it's disabled anyhow by the Makefile, but configure shouldn't *say* that
it is supported if it is not...

Closes: https://github.com/umlaeute/Gem/issues/38
do not check GLEW_... if not needed

should prevent crashes if glew was not initialized...

Closes: https://github.com/umlaeute/Gem/issues/40