cmake: misc improvements, add LIBSSH2_USE_PKGCONFIG option

- show platform flags (via curl).
- add `LIBSSH2_USE_PKGCONFIG` option to control whether to use
  `pkg-config` to find dependencies.
- set `.pc` names withing the Find modules.
- add `mbedcrypto` to `libssh2.pc` only when detected via `pkg-config`.
  Workaround for older mbedtls versions and non-CMake mbedTLS builds
  (as of mbedTLS 3.6.2) that don't emit an `mbedcrypto.pc` file.
- set header paths relative to the project root (tidy-up).
- use `-isystem` for crypto backend and zlib header paths.
  To match autotools.
- sync header path order with autotools.
- rename local variables to underscore-lowercase.
- minor tidy-ups.

Cherry-picked from #1484
Closes #1525

example/CMakeLists.txt

@@ -49,8 +49,8 @@ foreach(_example IN LISTS noinst_PROGRAMS)
   list(APPEND _example_targets ${_example})
   # to find generated header
   target_include_directories(${_example} PRIVATE
-    "${CMAKE_CURRENT_BINARY_DIR}/../src"
-    "../src")
+    "${PROJECT_BINARY_DIR}/src"
+    "${PROJECT_SOURCE_DIR}/src")
   target_link_libraries(${_example} ${LIB_SELECTED} ${LIBSSH2_LIBS})
   set_target_properties(${_example} PROPERTIES UNITY_BUILD OFF)
 endforeach()