add smarter checking for termcap support, which fixes a build problem when

ncurses is statically compiled with term info support, which is provided in
the statically compiled editline library that we are including. 
(issue #6948, original patch by casper, modified to use the features 
of AST_EXT_LIB)


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@25611 65c4cc65-6c06-0410-ace0-fbb531ad65f3