Fix configure.ac to abort when missing libraries.
Fix configure.ac to abort when missing libraries.

 - Modified configure.ac to abort generating the
   build system if there are missing libraries,
   and emit an error message telling the user
   which libraries need to be installed.
 - Also added a check for libsigar, with conditionalized
   behaviour for Mac OS X.  (libsigar can be missing
   on OS X, because we don't build the things it
   depends on anyway).

lees0414/EUproject,alco90/soml,mytestbed/oml,alco90/soml,alco90/soml,lees0414/EUproject,mytestbed/oml,mytestbed/oml,mytestbed/oml,alco90/soml,lees0414/EUproject,lees0414/EUproject,lees0414/EUproject,mytestbed/oml,alco90/soml