Use a tool dependency, not buildlink3, if PYTHON_FOR_BUILD_ONLY.

ok agc
Fix cross-build of ncurses by tool-depending on itself for tic.

ok agc
Fix cross-build of security/libgpg-error with CC_FOR_BUILD=NATIVE_CC.

ok agc
Fix x11/libXt cross-build.

The configure script, not just the makefile, needs to know about the
native C compiler.

ok agc
Fix bash cross-build on NetBSD by pre-answering autoconf run-tests.

ok agc
Fix devel/gmp cross-build by telling it about the native C compiler.

ok agc
Kludgily fix cross-build of net/socat on NetBSD.

The right fix is to thwap socat over the head for worrying about the
values of these constants at compile-time, but that's inexpedient...

ok agc
Fix cross-build of pkgtools/pkg_install.

Can't execute the newly built tools if they were cross-built.

(This is not especially useful until we do cross-bootstrap, though.)

ok agc
Fix net/libbind cross-build on NetBSD.

Instruct autoconf that the target will have /dev/random.

ok agc
Fix net/isc-dhcp4 cross-build on NetBSD.

Requires a hack to patch and configure the internal copy of bind for
cross-compilation.

Bump pkgrevision in case the way I did this hack changes anything.

ok agc
Fix zsh cross-build on NetBSD.

Patch it up to use compile-tests rather than run-tests in autoconf
for many of its tests (OS-independently).  Teach it about the few
remaining properties for NetBSD in particular.

ok agc
Split off py-libevent-rpcgen from devel/libevent.

Obviates libevent's python dependency, enabling cross-compilation of
libevent's dependents without having to cross-compile python itself.

ok agc
Fix poty in new py-libevent-rpcgen makefile.
...aaaaand set PKGNAME, not DISTNAME.