cmake: add LDAP Find module

[vszakats]

Move LDAP detection to its own Find module.

It supports pkg-config and the standard detection method used for
other dependencies, with version detection.

In curl CI it fixes LDAP detection in the OpenBSD job.

---

w/o whitespace: https://github.com/curl/curl/pull/15273/files?w=1

• rebase on build: omit certain deps from libcurl.pc unless found via pkg-config #15573.

[vszakats]

The native cmake detection gets weird and detects LDAP in the Apple SDK. That provides an ldap.h and lber.h, which are stubs including themselves (using <ldap.h>). This causes an infinite inclusion loop when compiling.

For now I opted out APPLE from using the native cmake detection, but this requires to keep the old manual LDAP detection logic, while it would be reasonable goal to drop that.

[vszakats]

After this patch, in CI, OpenLDAP is detected in the same jobs as before.

Except on OpenBSD where it wasn't detected before, but it is after:

ld: error: unable to find library -lldap

https://github.com/curl/curl/actions/runs/11695589111/job/32571261705#step:3:1014
→
https://github.com/curl/curl/actions/runs/11695455985/job/32570914117#step:3:149

[dg0yt]

"Traditional" linkers rely on libary order. REMOVE_DUPLICATES would remove the late duplicates (e.g. the last -lm), and this breaks linking with "traditional" linkers which rely on lib order.

Suggested change

	list(APPEND LDAP_LIBRARIES ${LDAP_LBER_LIBRARIES})
	list(REMOVE_DUPLICATES LDAP_LIBRARIES)
	list(REMOVE_ITEM LDAP_LIBRARIES ${LDAP_LBER_LIBRARIES})
	list(APPEND LDAP_LIBRARIES ${LDAP_LBER_LIBRARIES})

[vszakats]

Note to self that this pattern is also used for mbedtls and brotli (pending PR).

And also for Libs.private in libcurl.pc.

I guess these can be dropped. Or come up with a function dropping the early dupes.

(There is also the "overlinking" issue when linking shared libs with these deps. Could't find a solution for that.)

[vszakats]

Do you think this'd work?:

list(REVERSE LDAP_LIBRARIES)
list(REMOVE_DUPLICATES LDAP_LIBRARIES)
list(REVERSE LDAP_LIBRARIES)

[dg0yt]

I used to use this pattern, too.

In general, I just leave the duplicates untouched today (and patch out REMOVE_DUPLICATES). Different linkers may need different strategies (accoding to CMake). With Apple frameworks, you may accidentally remove -framework.