cmake: set SSL_BACKENDS

This groups all SSL backends into the feature "SSL" and sets the
SSL_BACKENDS analogue to configure.ac

Closes https://github.com/curl/curl/pull/3736

CMakeLists.txt

@@ -1179,10 +1179,7 @@ endfunction()
 
 # Clear list and try to detect available features
 set(_items)
-_add_if("WinSSL"        SSL_ENABLED AND USE_WINDOWS_SSPI)
-_add_if("OpenSSL"       SSL_ENABLED AND USE_OPENSSL)
-_add_if("DarwinSSL"     SSL_ENABLED AND USE_DARWINSSL)
-_add_if("mbedTLS"       SSL_ENABLED AND USE_MBEDTLS)
+_add_if("SSL"           SSL_ENABLED)
 _add_if("IPv6"          ENABLE_IPV6)
 _add_if("unix-sockets"  USE_UNIX_SOCKETS)
 _add_if("libz"          HAVE_LIBZ)
@@ -1245,6 +1242,18 @@ endif()
 string(REPLACE ";" " " SUPPORT_PROTOCOLS "${_items}")
 message(STATUS "Enabled protocols: ${SUPPORT_PROTOCOLS}")
 
+# Clear list and collect SSL backends
+set(_items)
+_add_if("WinSSL"        SSL_ENABLED AND USE_WINDOWS_SSPI)
+_add_if("OpenSSL"       SSL_ENABLED AND USE_OPENSSL)
+_add_if("DarwinSSL"     SSL_ENABLED AND USE_DARWINSSL)
+_add_if("mbedTLS"       SSL_ENABLED AND USE_MBEDTLS)
+if(_items)
+  list(SORT _items)
+endif()
+string(REPLACE ";" " " SSL_BACKENDS "${_items}")
+message(STATUS "Enabled SSL backends: ${SSL_BACKENDS}")
+
 # curl-config needs the following options to be set.
 set(CC                      "${CMAKE_C_COMPILER}")
 # TODO probably put a -D... options here?