build: omit certain deps from libcurl.pc unless found via pkg-config

The idea of linking dependencies found to `libcurl.pc` turns out not
to work in practice in some cases.

Specifically: gss, ldap, mbedtls, libmsh3, rustls

A `.pc` may not work or be missing for a couple of reasons:
- not all build methods generate it: mbedTLS, Rustls
- generated file is broken: msh3
  Ref: https://github.com/nibanks/msh3/pull/225
- installed package flavour isn't shipping with one:
  FreeBSD GSS, OmniOS LDAP, macOS LDAP

The effect of such issues shall be subtle in theory, because
`libcurl.pc` normally lists these dependencies in the `Requires.private`
section meant for static linking. But, e.g. `pkg-config --exists`
requires these to be present, and builds sometimes use this check
regardless of build type. This bug is not present in `pkgconf`; it only
checks for them when `--static` is also passed.

Fix these by adding affected `.pc` references to `libcurl.pc` only when
we detected the dependency via `pkg-config`.

There are a few side-effects of this solution:
- references are never added for dependencies where curl doesn't
  implement `pkg-config` detection. These are:
  - autotools: ldap, mbedtls, msh3
  - cmake: ldap (pending #15273)
- generated `libcurl.pc` depends on the build-time environment.
- generated `libcurl.pc` depends on curl build tool (cmake, autotools).
- generated `libcurl.pc` depends on curl build implementation details.

Make an exception for GNU GSS, where I blindly guess that `gss.pc` is
always available, as no issues were reported.

Other, not mentioned, dependencies continue to be added regardless
of the detection method.

Reported-by: Harmen Stoppels, Thomas, Daniel Engberg, Andy Fiddaman
Fixes #15469
Fixes #15507
Fixes #15535
Fixes https://github.com/curl/curl/pull/15163#issuecomment-2473358444
Closes #15573

acinclude.m4

@@ -565,7 +565,9 @@ AC_DEFUN([CURL_CHECK_LIBS_LDAP], [
       else
         LIBS="$curl_cv_ldap_LIBS $curl_cv_save_LIBS"
       fi
-      LIBCURL_PC_REQUIRES_PRIVATE="ldap $LIBCURL_PC_REQUIRES_PRIVATE"
+      if false; then
+        LIBCURL_PC_REQUIRES_PRIVATE="ldap $LIBCURL_PC_REQUIRES_PRIVATE"
+      fi
       AC_MSG_RESULT([$curl_cv_ldap_LIBS])
       ;;
   esac