add pcre2 support

[elliefm]

• Adds pcre2 support based on Update to pcre2 #4545, but without removing legacy pcre support
• Updates pcre/pcre2 checks to use pkg-config, which should fix 3.4.6: can't find pcre in /usr/include/pcre #4711
• Prefers pcre2 over pcre if both are available

Fixes: #3861 #4711

[elliefm]

@elliefm When backporting, leave out the "prefer pcre2..." commit

[rsto]

Builds and unit tests clean on my build

"pcre": false,
"pcre2": true,

[ksmurchison]

Builds and unit tests clean on my build

"pcre": false,
"pcre2": true,

Same here.

[elliefm]

@rsto I've added upgrade instructions to the changes file to clarify the pcre/pcre2/pcre3 situation, to explain how to make configure use the older one when both are present (otherwise it will prefer the newer one), and to explain how to deal with a situation where pcre2 is half-installed already (but not enough for Cyrus to use). I think this should address your issues deploying this PR to Fastmail -- in a generic way that will be useful to non-Fastmail deployments.

Sorry I missed this earlier. My bullseye VM already had libpcre2-dev installed as a dependency for some other thing, so in my testing I didn't need to do anything in particular to switch between versions. It hadn't occurred to me that a system might have enough of pcre2 to detect but not enough of it to actually use.

In Fastmail-specific terms (but please read the generic instructions too): I think you need to modify mkdebian.pl to either:

• replace libpcre3-dev with libpcre2-dev in Build-Depends: (as well as whatever else it takes to get that package and its dependencies actually installed onto systems); OR
• add --disable-pcre2 to the ./configure line under build:.

We can get away with staying on the old (unsupported, unmaintained) pcre (aka pcre3) for the time being, but we will be forced to upgrade to pcre2 when we upgrade to Bookworm, so IMO we might as well get it over with now.

[rsto]

Thanks, looks good to me.

[ksmurchison]

LGTM, with a small nit

[elliefm]

@elliefm when you get to backporting this, the 3.4 conversion is already done in #4738 (just needs release notes as usual)

[elliefm]

@elliefm any version that this gets backported to will also need #4741

[elliefm]

@elliefm and also #4749