MINOR: compiler: add a simple macro to concatenate resolved strings

MINOR: compiler: add a simple macro to concatenate resolved strings

It's often useful to be able to concatenate strings after resolving
them (e.g. __FILE__, __LINE__ etc). Let's just have a CONCAT() macro
to do that, which calls _CONCAT() with the same arguments to make
sure the contents are resolved before being concatenated.

(cherry picked from commit bb4addabb742f2305ad6667ed42ebb12f5df2af3)
Signed-off-by: Willy Tarreau <w@1wt.eu>
(cherry picked from commit af1e0b247e64678927793e9cd6e10907b0eebb27)
Signed-off-by: Willy Tarreau <w@1wt.eu>