cmake: speed up warning option detection

[vszakats]

This patch makes CMake configuration faster by dropping feature checks
for each individual "picky" C/C++ compiler warning options and instead
enable them based on compiler version. It also allows to add options using
the old, feature-check method.

I've implemented this for libssh2 [1] and curl [2] earlier this year.
This patch is based on those, by adapting it to the warning options used
by ngtcp2.

It brings CMake configuration time down from 15 to 5 seconds on my local
machine.

Special mention for -Wextended_offsetof: I could find no evidence that
this warning option was ever implemented in clang (or gcc), so this patch
deletes it.

[1] libssh2
libssh2/libssh2@ec0feae
libssh2/libssh2#952

[2] curl
curl/curl@9c543de
curl/curl#10973

If this is welcome, I have a similar patch for nghttp2 and nghttp3 as well.

[tatsuhiro-t]

Thank you for PR. Could you change the license to MIT and rpelace the license header with the following one?

# ngtcp2
#
# Copyright (c) 2023 ngtcp2 contributors
#
# Permission is hereby granted, free of charge, to any person obtaining
# a copy of this software and associated documentation files (the
# "Software"), to deal in the Software without restriction, including
# without limitation the rights to use, copy, modify, merge, publish,
# distribute, sublicense, and/or sell copies of the Software, and to
# permit persons to whom the Software is furnished to do so, subject to
# the following conditions:
#
# The above copyright notice and this permission notice shall be
# included in all copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
# EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
# NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE
# LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION
# OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION
# WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

[vszakats]

I've updated the licenses.

[tatsuhiro-t]

Thank you! Merged now.