BUILD: compiler: undefine the CONCAT() macro if already defined

BUILD: compiler: undefine the CONCAT() macro if already defined

As Ilya reported in issue #2911, the CONCAT() macro breaks on NetBSD
which defines its own as __CONCAT() (which is exactly the same). Let's
just undefine it before ours to fix the issue instead of renaming, but
keep ours so that we don't have doubts about what we're running with.

Note that the patch introducing this breaking change was backported
to 3.0.