Remove definition of the GCC3 variable. It was introduced in rev.

Remove definition of the GCC3 variable. It was introduced in rev.
1.50 to help out with the GCC 2 to GCC 3 transition and it became
obsolete when C flags compatible with GCC 3.x became the default.
With GCC 4 in the tree this variable (i.e. GCC3) is beyond bogus
because it causes confusion when looking for the newly introduced
WITH_GCC3 option that helps the GCC 3 -> GCC 4 bump.