Fix buildworld. Document TOOLS_PREFIX and USRDATA_PREFIX, improve INCLUDEDIR

Fix buildworld.  Document TOOLS_PREFIX and USRDATA_PREFIX, improve INCLUDEDIR
documentation.  Modify bsd.incs.mk to not install header files if BOOTSTRAPPING
is set (for buildworld), and change lex to install its C++ header file in
${INCLUDEDIR}/c++ instead of ${INCLUDEDIR}/g++.  Set DESTDIR for BMAKEENV,
set BOOTSTRAPPING for XMAKE (cross build tools).  Note that DESTDIR is set
in the bootstrap-tools: target, this will be removed in a later commit.