php8: link PCRE2 JIT usage to actual library configuration

php8: link PCRE2 JIT usage to actual library configuration

libpcre2's JIT support is controlled with a compile-time flag.
When PHP is compiled against it (external libpcre2 - not PHP's bundled
version) as in our use-case, then it tries to auto-detect whether
JIT support is enalbed or not - but this is done based on a guess
when cross-compiling.

So instead of guessing, we need to tell the actual OpenWrt configuration
of libpcre2 to PHP.

Signed-off-by: Michael Heimpold <redacted>