x11-fm/polo: respect CFLAGS to unbreak the build on recent -CURRENT

Commit c57cd4390a27 had pacified one particular warning which often
breaks Vala code compilation with Clang 16, thus correct fix is not
to force an older compiler, but to pass down our CFLAGS to valac(1)
together with -v option to make the actual compiler call visible in
the build logs.

Fixes: 09e56dc8e568