Conditionally include some standard -L and -I directories.
Fix MPI check. AC_PROG_CC_MPI replaces AC_PROG_CC so we must not run
the latter first.
Remove obsoleted stuff from para-best.pl
Fix X32 build. There were several issues but nothing major - except maybe
we can't add '-m32' or '-m64' from configure. That sort of thing must be
left to the user.

./configure CFLAGS=-mx32 ASFLAGS=-mx32 LDFLAGS=-mx32 --disable-mpi
Add shorthand "--with-arch=opt" which adds opt to CFLAGS, ASFLAGS and
LDFLAGS. The best example might be "--with-arch=-m32" altough many other
uses are possible.

Do not enable MPI automatically, because users might be scared by
personal firewalls complaining about "network activity".

Add --disable-nss option. This might be needed for cross compiling since
pkg-check is not suitable for that.