specified by passing the XCC, XCXX, and XCPP variables (corresponding to
CC, CXX, and CPP) to buildworld/buildkernel. The compiler must be clang
or be configured to target the appropriate architecture.
To speed build times, if XCC is an absolute path or
WITHOUT_CROSS_COMPILER is defined then no cross compiler will be built
during the cross-tools stage.
Limited documentation of this feature can currently be found at:
https://wiki.freebsd.org/ExternalToolchain
This functionality should be considered experimental and is subject to
change without notice.
Sponsored by: DARPA, AFRL
Discussed with: imp, sjg
