- Remove a FreeBSD-specific hack from 'find_a_file()' that hardcodes a
single directory for includes.
- Change the C++ directory entries to honor --sysroot if it is set.
- Don't define CROSS_INCLUDE_DIR for the cross compiler. Instead, set
TARGET_SYSTEM_ROOT to point to WORLDTMP and always define
- Change STANDARD_INCLUDE_DIR and the C++ include directories to just
start with "/usr" always. The compiler will prepend the sysroot when
doing cross-builds. GCC_INCLUDE_DIR (which contains headers that ship
with the compiler such as intrinsincs rather than OS-supplied headers)
retains hardcoded to look in TOOLS_PREFIX.