libXext/buildlink3.mk, now that it is included there.
Leave the places where its API version is set or variables from it
are used directly (about 3 packages).
The previous changeset, "Fix issues with new GCC" overwrote the existing
patch-ab which dealt with pre-existing alloca definitions. Incorporate
the old patch-ab into the current patch-ab to fix regression.
since they always need a C compiler, even when the source code is
completely in C++.
For some other packages, stated in the comment that a C compiler is
really not needed.