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.

(cherry picked from commit 4ec5509541e29956107011d45c971a9fe3a430f1)
Signed-off-by: Aurelien DARRAGON <adarragon@haproxy.com>
(cherry picked from commit bf3252c1d48a63cccee17793b1f24e01409eaf01)
Signed-off-by: Aurelien DARRAGON <adarragon@haproxy.com>