cmake: avoid setting custom property on built-in interface targets

In some cases `ZLIB::ZLIB` and/or `OpenSSL::Crypto` may be aliases,
which prevents setting a libssh2-specific property (.pc module name)
in them:
```
CMake Error at [...]/src/CMakeLists.txt:... (set_target_properties):
  set_target_properties can not be used on an ALIAS target.
```

This can happen when doing "superbuilds" with classic zlib or zlib-ng,
which define `ZLIB::ZLIB` on their own, but as an alias, unlike CMake
does with the canonical `ZLIB::ZLIB` target.

Fix by special-casing these built-in targets and manually converting
them to .pc module names, without using the targets themselves
to carry this information throughout libssh2's internal build logic.

A side-effect of this change is that `zlib` is now present in libssh2.pc
when zlib is an indirect dependency via a crypto backend (OpenSSL or
wolfSSL). Before this patch it only appeared there when enabling zlib
explicitly for libssh2.

Ref: https://github.com/curl/curl/pull/20316
Follow-up to df0563a857 #1535

Closes #1789

CMakeLists.txt

@@ -424,7 +424,6 @@ if(CRYPTO_BACKEND STREQUAL "OpenSSL" OR NOT CRYPTO_BACKEND)
     set(CRYPTO_BACKEND "OpenSSL")
     set(CRYPTO_BACKEND_DEFINE "LIBSSH2_OPENSSL")
     list(APPEND LIBSSH2_LIBS OpenSSL::Crypto)
-    set_target_properties(OpenSSL::Crypto PROPERTIES INTERFACE_LIBSSH2_PC_MODULES "libcrypto")
 
     if(WIN32)
       # Statically linking to OpenSSL requires crypt32 for some Windows APIs.

src/CMakeLists.txt

@@ -60,7 +60,6 @@ if(ENABLE_ZLIB_COMPRESSION)
   find_package(ZLIB REQUIRED)
 
   list(APPEND LIBSSH2_LIBS ZLIB::ZLIB)
-  set_target_properties(ZLIB::ZLIB PROPERTIES INTERFACE_LIBSSH2_PC_MODULES "zlib")
   list(APPEND _libssh2_definitions "LIBSSH2_HAVE_ZLIB")
 endif()
 
@@ -283,7 +282,13 @@ if(NOT LIBSSH2_DISABLE_INSTALL)
       if(NOT _libname AND NOT _libs AND NOT _libdirs)
         message(WARNING "Bad lib in library list: ${_lib}")
       endif()
-      get_target_property(_modules "${_lib}" INTERFACE_LIBSSH2_PC_MODULES)
+      if(_lib STREQUAL OpenSSL::Crypto)
+        set(_modules "libcrypto")
+      elseif(_lib STREQUAL ZLIB::ZLIB)
+        set(_modules "zlib")
+      else()
+        get_target_property(_modules "${_lib}" INTERFACE_LIBSSH2_PC_MODULES)
+      endif()
       if(_modules)
         list(APPEND LIBSSH2_PC_REQUIRES_PRIVATE "${_modules}")
       endif()