lib: Add PCRE2 support

lib: Add PCRE2 support

Some results:

```
====
PCRE
====
% ./a.out "^65001" "65001"
comparing: ^65001 / 65001

ret status: 0
[14:31] donatas-pc donatas /home/donatas
% ./a.out "^65001_" "65001"
comparing: ^65001_ / 65001

ret status: 0

=====
PCRE2
=====
% ./a.out "^65001" "65001"
comparing: ^65001 / 65001

ret status: 0
[14:30] donatas-pc donatas /home/donatas
% ./a.out "^65001_" "65001"
comparing: ^65001_ / 65001

ret status: 1
```

Seems that if using PCRE2, we need to escape outer `()` chars and `|`. Sounds
like a bug.
But this is only with some older PCRE2 versions. With >= 10.36, I wasn't able
to reproduce this, everything is fine and working as expected.

Adding _FRR_PCRE2_POSIX definition because pcre2posix.h does not have
include's guard.

Signed-off-by: Donatas Abraitis <donatas@opensourcerouting.org>