Properly document AST_EXT_TOOL_CHECK() and use it to check for

Properly document AST_EXT_TOOL_CHECK() and use it to check for
NETSMP and GTK (GTK is not used thoug).

AST_EXT_TOOL_CHECK() could be used for checking curl status
as well, perhaps with a small addition because we currently seem
to require a curl version greater than X.Y.Z

Add a NETSMP_INCLUDE entry in makeopts.in

We don't have yet any macros for using pkg-config to check
for a specific package (right now there is only gtk2+
in the category).

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@93898 65c4cc65-6c06-0410-ace0-fbb531ad65f3