build: pmdasnmp makefile needs conditional disablement
build: pmdasnmp makefile needs conditional disablement

Make pmdasnmp like the database PMDAs, such that when its not been
enabled in the build (via configure.ac) we don't still install it.

wuliming/pcp,wuliming/pcp,adfernandes/pcp,wuliming/pcp,wuliming/pcp,wuliming/pcp,adfernandes/pcp,adfernandes/pcp,wuliming/pcp,adfernandes/pcp,adfernandes/pcp,adfernandes/pcp,wuliming/pcp,adfernandes/pcp,wuliming/pcp,adfernandes/pcp