cmake: drop support for CMake 3.17 and older

Require CMake 3.18 (2020-07-15) or newer, up from 3.7 (2016-11-11)
prior to this patch.

This requirement also applies to the distributed `curl-config.cmake`.

To allow dropping compatibility code maintained for old versions, and to
use features which were unpractical in separate code paths. Also to make
testing, documentation and development easier, CI builds faster due to
CMake performance improvements over time. (e.g. integration tests on
macOS run 8x faster (10 minutes is now under 1.5m) in CI, 2.5x faster on
Windows.)

CMake offers pre-built binaries for major platforms. They work without
an install step, just by unpacking and pointing the cmake command to
them. Making upgrades easy in many cases:
https://cmake.org/download/
https://cmake.org/files/
https://github.com/Kitware/CMake/releases

CMake 3.18 brings these feature as generally available when building or
consuming curl/libcurl:

LTO support, improved performance, `pkg-config` and interface target
support, `OBJECT` target (for faster libcurl builds), modern invocation
with `-S`/`-B` options, better support for custom linker options,
FetchContent, `GnuTLS::GnuTLS` target, `--verbose` and `--install`
options, `CMAKE_GENERATOR` env, last but not least unity mode and Ninja
generator.

For maximum build speed, use:
`-DCMAKE_UNITY_BUILD=ON -DCURL_DROP_UNUSED=ON`

As for deprecations, C++11 is required to build CMake itself, which may
be a limit on some platforms. autotools continues to cover them.

Follow-up to 9bcdfb3809 #20408
Follow-up to a7c974e038 #19902
Follow-up to dfbe035c8b #10161
Discussion: https://github.com/curl/curl/discussions/18704

Closes #20407

appveyor.sh

@@ -70,28 +70,17 @@ if [ -n "${CMAKE_GENERATOR:-}" ]; then
     [ "${_chkprefill}" = '_chkprefill' ] && options+=' -D_CURL_PREFILL=OFF'
     [[ "${CMAKE_GENERATE:-}" = *'-A ARM64'* ]] && SKIP_RUN='ARM64 architecture'
     [[ "${CMAKE_GENERATE:-}" = *'-DCURL_USE_OPENSSL=ON'* ]] && options+=" -DOPENSSL_ROOT_DIR=${openssl_root_win}"
-    if [ "${APPVEYOR_BUILD_WORKER_IMAGE}" = 'Visual Studio 2013' ]; then
-      mkdir "_bld${_chkprefill}"
-      cd "_bld${_chkprefill}"
-      options+=' ..'
-      root='..'
-    else
-      options+=" -B _bld${_chkprefill}"
-      options+=' -DCMAKE_VS_GLOBALS=TrackFileAccess=false'
-      options+=' -DCMAKE_UNITY_BUILD=ON'
-      root='.'
-    fi
-    # CMAKE_GENERATOR env requires CMake 3.15+, pass it manually to make it work with older versions.
     # shellcheck disable=SC2086
-    time cmake -G "${CMAKE_GENERATOR}" \
-      -DENABLE_DEBUG=ON -DCURL_WERROR=ON \
+    time cmake -B "_bld${_chkprefill}" \
+      -DENABLE_DEBUG=ON \
+      -DCMAKE_UNITY_BUILD=ON -DCURL_WERROR=ON \
+      -DCMAKE_VS_GLOBALS=TrackFileAccess=false \
       -DCURL_STATIC_CRT=ON \
       -DCURL_DROP_UNUSED=ON \
       -DCURL_USE_SCHANNEL=ON -DCURL_USE_LIBPSL=OFF \
-      ${options} \
       ${CMAKE_GENERATE:-} \
-      || { cat "${root}"/_bld/CMakeFiles/CMake* 2>/dev/null; false; }
-    [ "${APPVEYOR_BUILD_WORKER_IMAGE}" = 'Visual Studio 2013' ] && cd ..
+      ${options} \
+      || { cat _bld/CMakeFiles/CMake* 2>/dev/null; false; }
   done
   if [ -d _bld_chkprefill ] && ! diff -u _bld/lib/curl_config.h _bld_chkprefill/lib/curl_config.h; then
     cat _bld_chkprefill/CMakeFiles/CMake* 2>/dev/null || true