This was done following the steps on in the `NON-AUTOTOOLS-BUILD` file
found in the root of the pcre2 repository:
https://github.com/PhilipHazel/pcre2/blob/pcre2-10.40/NON-AUTOTOOLS-BUILD

CMake now has to specify each pcre file individually, as some are not meant to be compiled by themselves.

These were necessary in pcre1 but are handled by PCRE2_CODE_UNIT_WIDTH
in pcre2

Since pcre 10.30, pcre2_match no longer uses recursive function calls,
so this setting should no longer be needed.

This avoids the memory leak caused by having to configure this setting
using a context structure allocated at run time.

This is the pcre2 equivalent of the PCRE_JAVASCRIPT_COMPAT flag we used
previously. Compare bullet point 4 of the pcre2 compatibility docs:
https://www.pcre.org/current/doc/html/pcre2compat.html
to bullet point 5 of the pcre1 compatibility docs:
https://www.pcre.org/original/doc/html/pcrecompat.html

`PCRE2_ALT_BSUX`, `PCRE2_ALLOW_EMPTY_CLASS`,
and `PCRE2_MATCH_UNSET_BACKREF`
are equivalent to the old `PCRE_JAVASCRIPT_COMPAT`.

See:
https://lists.exim.org/lurker/message/20150105.162835.0666407a.en.html