Honor the --with-${library}=path for AST_EXT_TOOL_CHECK

(closes issue #16991)
Reported by: pprindeville
Patches: 
      with_netsnmp.patch.txt uploaded by twilson (license 396)
Tested by: twilson

Review: https://reviewboard.asterisk.org/r/739/


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@272254 65c4cc65-6c06-0410-ace0-fbb531ad65f3