notmuch-new: Only install SIGALRM if not running under gdb

I felt sorry for Carl trying to step through an exception from xapian
and suffering from the SIGALARMs..

We can detect if the user launched notmuch under a debugger by either
checking our cmdline for the presence of the gdb string or querying if
valgrind is controlling our process. For the latter we need to add a
compile time check for the valgrind development library, and so add the
initial support to build Makefile.config from configure.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Carl Worth <cworth@cworth.org>
[ickle: And do not install the timer when under the debugger]