Replace some cpp with autoconf.

Replace some cpp with autoconf.

* configure.in (--with-mmdf, --with-mail-unlink):
New options, off by default.
(--with-mailhost): New option to set default POP host.
(LIBXPM, LIBJPEG, LIBPNG, LIBTIFF, LIBGIF, LIBGPM, LIBS_MAIL)
(LIBHESIOD, LIBRESOLV, COM_ERRLIB, CRYPTOLIB, KRB5LIB, DESLIB, KRB4LIB):
New variables, substituted in Makefiles.
(try_libungif, ac_gif_lib_name): Replace with HAVE_GIF=maybe, LIBGIF.
(LIBGIF): Use AC_SUBST rather than AC_DEFINE.
(HAVE_LIBMAIL, HAVE_LIBLOCKFILE, HAVE_LIBCOM_ERR, HAVE_LIBCRYPTO)
(HAVE_LIBK5CRYPTO, HAVE_LIBKRB5, HAVE_LIBDES425, HAVE_LIBDES)
(HAVE_LIBKRB4, HAVE_LIBKRB): New AC_DEFINEs.

* lib-src/Makefile.in (KRB4LIB, DESLIB, KRB5LIB, CRYPTOLIB, COM_ERRLIB)
(HESIODLIB, LIBS_MAIL): Set using autoconf rather than cpp.
(BASE_CFLAGS): Remove (identical to CPP_CFLAGS).

* src/Makefile.in (XFT_LIBS, LIBXPM, LIBJPEG, LIBPNG, LIBTIFF, LIBGIF)
(LIBGPM, LIBRESOLV): Set using autoconf rather than cpp.