Add TAP tests for client programs

Reviewed-by: Pavel Stěhule <pavel.stehule@gmail.com>
Reviewed-by: Erik Rijkers <er@xs4all.nl>
Add @configure_input@ marker to Makefile.global.in

That way, when looking at Makefile.global, we don't get confused by the
comment that claims that this is Makefile.global.in.
Have lcov call the right gcov

By default, lcov will call whatever gcov it can find in the path.  But
if the user has specified a different gcov to configure, this could be
incompatible.  So tell lcov explicitly with an option which gcov program
to call.