ports/77707: Fix for ports/77403 introduced circular dependency in ports tree

[freebsd at chillt.de (Bartosz Fabianowski)]

> So we've overloaded the meaning of the variable.  At least this is
> what I recall.

I see. It doesn't seem to have been announced anywhere big time, so 
missed that information. Sorry.

While lang/gcc32 will build just fine without USE_GCC, the compiler used 
in this case (cc) will complain a lot about GCC extensions being used. 
So that's not a good idea. It seems to me that the following might be a 
proper fix:

bsd.gcc.mk should try to satisfy the USE_GCC with the system compiler 
first and only if that does not match the version requested, it should 
try port versions. This way, USE_GCC= 2.7+ would take on the same 
meaning as the old USE_GCC= X in that it simply requests for system GCC 
to be used. At the same time, things such as USE_GCC= 3.1 would continue 
to work because if the system compiler isn't 3.1, bsd.gcc.mk would keep 
looking for an acceptable GCC.

- Bartosz