Correct C++ header handling for gcc2 and lex.

Correct C++ header handling for gcc2 and lex.

gcc2 used a "beforeinstall" target instead of the standard bsd.incs.mk way.
Therefore certain headers weren't correctly installed when doing an
installincludes or "make includes" from the src root. The cc1plus part was
still installed to the old location and that broke e.g. textproc/jade.

lex installed its C++ interface still to /usr/include/g++, until a decision
about a generic C++ header location is made, a version for both system
compilers is installed.