CMake: clean up and improve build procedures

- remove check for unsupported old CMake versions

- do not link to c-ares library twice

- modernize custom Find modules

    - FindLibSSH2:
        - pass version to FPHSA to show it in the output
        - use LIBSSH2_VERSION define to extract the version number in
          one shot. This variable exists in the header for 10 years.
        - remove unneeded code

    - FindNGHTTP2.cmake:
        - drop needless FPHSA argument
        - mark found variables as advanced

    - FindNSS.cmake:
        - show version number

    - FindCARES.cmake:
        - drop default paths
        - use FPHSA instead of checking things by hand

- remove needless explict variable dereference

- simplify count_true()

- allow all policies up to version 3.16 to be set to NEW

- do not rerun check for -Wstrict-aliasing=3 every time

In contrast to every other compiler flag this has a = in it, which CMake
can't have in a variable name.

- only read the interesting strings from curlver.h

Reviewed-by: Peter Wu

Closes https://github.com/curl/curl/pull/4975

CMake/FindNGHTTP2.cmake

@@ -10,9 +10,9 @@ find_package_handle_standard_args(NGHTTP2
     REQUIRED_VARS
       NGHTTP2_LIBRARY
       NGHTTP2_INCLUDE_DIR
-    FAIL_MESSAGE
-      "Could NOT find NGHTTP2"
 )
 
 set(NGHTTP2_INCLUDE_DIRS ${NGHTTP2_INCLUDE_DIR})
 set(NGHTTP2_LIBRARIES ${NGHTTP2_LIBRARY})
+
+mark_as_advanced(NGHTTP2_INCLUDE_DIRS NGHTTP2_LIBRARIES)