BUILD: makefile: properly report USE_PCRE/USE_PCRE2 in features

BUILD: makefile: properly report USE_PCRE/USE_PCRE2 in features

The PCRE/PCRE2 CFLAGS forcefully add -DUSE_PCRE or -DUSE_PCRE2 because
we want that USE_STATIC_PCRE or USE_PCRE_JIT implicitly enables them.
However, doing it this way is incorrect because the option is not visible
in BUILD_FEATURES, and for example, some regtests depending on such
features (such as map_redirect.vtc) would be skipped if only the static
or jit versions are enabled.

The correct way to do this is to always set USE_PCRE feature for such
variants instead of adding the define.

This could almost be backported but would require to backport other
makefile patches and likely only has effects on the reg-tests at the
moment, so it's probably not worth the hassle.