cmake: publish/check supported protocols/features via CURLConfig.cmake

Via these variables, as lists:
- `CURL_SUPPORTED_PROTOCOLS`
- `CURL_SUPPORTED_FEATURES`

As individual flags:
- `CURL_SUPPORTS_<protocol/feature>` = `TRUE`

Also:
- set `CURL_VERSION_STRING` which was missing when using
  `find_package(CURL CONFIG)` or
  `find_package(CURL NO_MODULE)`.
- set `CURL_<prototol/feature>_FOUND` for compatibility.
- show full list of missing but required `COMPONENTS`.

Assisted-by: Derek Huang
Fixes #15854
Closes #15858

CMake/curl-config.cmake.in

@@ -39,7 +39,6 @@ if("@HAVE_LIBZ@")
 endif()
 
 include("${CMAKE_CURRENT_LIST_DIR}/@TARGETS_EXPORT_NAME@.cmake")
-check_required_components("@PROJECT_NAME@")
 
 # Alias for either shared or static library
 if(NOT TARGET @PROJECT_NAME@::libcurl)
@@ -47,5 +46,32 @@ if(NOT TARGET @PROJECT_NAME@::libcurl)
 endif()
 
 # For compatibility with CMake's FindCURL.cmake
+set(CURL_VERSION_STRING "@CURLVERSION@")
 set(CURL_LIBRARIES @PROJECT_NAME@::libcurl)
 set_and_check(CURL_INCLUDE_DIRS "@PACKAGE_CMAKE_INSTALL_INCLUDEDIR@")
+
+set(CURL_SUPPORTED_PROTOCOLS "@CURL_SUPPORTED_PROTOCOLS_LIST@")
+set(CURL_SUPPORTED_FEATURES "@CURL_SUPPORTED_FEATURES_LIST@")
+
+foreach(_item IN LISTS CURL_SUPPORTED_PROTOCOLS CURL_SUPPORTED_FEATURES)
+  set(CURL_SUPPORTS_${_item} TRUE)
+endforeach()
+
+set(_missing_req "")
+foreach(_item IN LISTS CURL_FIND_COMPONENTS)
+  if(CURL_SUPPORTS_${_item})
+    set(CURL_${_item}_FOUND TRUE)
+  elseif(CURL_FIND_REQUIRED_${_item})
+    list(APPEND _missing_req ${_item})
+  endif()
+endforeach()
+
+if(_missing_req)
+  string(REPLACE ";" " " _missing_req "${_missing_req}")
+  if(CURL_FIND_REQUIRED)
+    message(FATAL_ERROR "CURL: missing required components: ${_missing_req}")
+  endif()
+  unset(_missing_req)
+endif()
+
+check_required_components("@PROJECT_NAME@")

CMakeLists.txt

@@ -2042,6 +2042,7 @@ curl_add_if("WSS"           NOT CURL_DISABLE_WEBSOCKETS AND _ssl_enabled)
 if(_items)
   list(SORT _items)
 endif()
+set(CURL_SUPPORTED_PROTOCOLS_LIST "${_items}")
 string(REPLACE ";" " " SUPPORT_PROTOCOLS "${_items}")
 string(TOLOWER "${SUPPORT_PROTOCOLS}" _support_protocols_lower)
 message(STATUS "Protocols: ${_support_protocols_lower}")
@@ -2095,6 +2096,7 @@ if(_items)
     list(SORT _items)
   endif()
 endif()
+set(CURL_SUPPORTED_FEATURES_LIST "${_items}")
 string(REPLACE ";" " " SUPPORT_FEATURES "${_items}")
 message(STATUS "Features: ${SUPPORT_FEATURES}")
 
@@ -2349,10 +2351,13 @@ if(NOT CURL_DISABLE_INSTALL)
     ${_generated_version_config}")
 
   # Consumed custom variables:
+  #   CURLVERSION
   #   LIB_SELECTED
   #   TARGETS_EXPORT_NAME
   #   USE_OPENSSL
   #   HAVE_LIBZ
+  #   CURL_SUPPORTED_FEATURES_LIST
+  #   CURL_SUPPORTED_PROTOCOLS_LIST
   configure_package_config_file("CMake/curl-config.cmake.in"
     "${_project_config}"
     INSTALL_DESTINATION ${_install_cmake_dir}