Remove support for COMPILER_VERSION == gcc2.
Change the logic depending upon COMPILER_VERSION everywhere, to assume gcc4
is the norm and to explicitely test for gcc3 when a different behaviour
is required.
No functional change intended. Be sure to `make install' in share/mk before
attempting to do anything.

Switch to using COMPILER_VERSION instead of USE_GCC3, allows for more flexible
complier picking, eg supporting gcc2, gcc3, and gcc4. based on diff by Marco,
with fixes from espie@.
ok espie@ general mumbling of approval of others.

branches: 1.1.1;
Initial revision