Add cross/libtool-base package (cross-libtool-base-${MACHINE_ARCH}).

This is a provisional kludge to enable cross-compilation of packages
using libtool.  It requires manual intervention: you must build it as
a target package, and then install the target package on the host
with `pkg_add -m ${TARGET_ARCH}'.

ok agc
Set NATIVE_CC/CC, NATIVE_CXX/CXX, and NATIVE_LD/LD for cross-builds.

The NATIVE_xyz versions are for packages that build tools that they
run natively but don't end up in the final product.

This is a provisional scheme -- it should be replaced eventually by
something more principled.

ok agc
Use --sysroot in cpp/gcc/gxx cross-compiler wrappers.

Now that NetBSD supports --sysroot we don't need to mess with the
partial measures of -nostdinc, -isystem, &c.

ok agc
Split BUILD_DEPENDS into TOOL_DEPENDS and BUILD_DEPENDS in mk/.

Build depends are target packages that are needed at build-time for,
e.g., static libraries to link against, header files to include, &c.
Tool depends are native packages that are needed at build-time for,
e.g., compilers/linkers/&c. to run.

ok agc
Rename HOWTO-crosscompile to HOWTO-distcc for clarity.

ok agc
Remove the now-outdated HOWTO-crosscompile-xorg.
Add $NetBSD$ tag to HOWTO-distcc.
Add some notes on how to use and develop cross-compiled packages.

ok agc
Fix pathnames and NM in libtool for cross-compilation.

Forgot to commit this alongside the recent cross-libtool changes.

ok agc