Clean up configure-time library handling a bit.

Clean up configure-time library handling a bit.

This patch was inspired by emacs-24 2014-04-09T13:37:49Z!sdl.web@gmail.com, which fixed
a bug due to sloppy library handling in 'configure'.
* configure.ac (LIB_MATH, LIB_PTHREAD, LIBXMU):
Use AC_SEARCH_LIBS instead of AC_CHECK_LIB as per Autoconf manual.
(LIB_MATH, LIB_PTHREAD, HAVE_X11, IMAGEMAGICK_LIBS, GTK_LIBS)
(DBUS_LIBS, LIBXMU, XFT_LIBS, LIBXSM, LIBXML2_LIBS, LIBS_MAIL)
(with_kerberos):
Don't let the library choice infect $LIBS.
(dnet_ntoa, cma_open): Remove obsolete tests.
(emacs_pthread_function): Probe for pthread_kill, not pthread_self,
as that's a bit more selective on GNU/Linux.
(LIBXEXT): Remove.
(touchlock): Test for existence when $LIBS_MAIL is in use.
(AC_CHECK_FUNCS): Use only $LIB_MATH in addition to $LIBS
when testing for typical functions like accept4, lrand48.
(random, rint): Remove obsolete HP-UX 9 A.09.05 test.