A project being built entirely statically will call pkg-config with
--static, which utilises the Libs.private field. Conversely it will
not use --static when not being built entirely statically, even if
there is only a static build of libcurl available. This will most
likely cause the build to fail due to underlinking unless we merge the
Libs fields.

Consider that this is what the Meson build system does when it
generates pkg-config files.

I have also reflected this in the --libs argument of curl-config even
though REQUIRE_LIB_DEPS always seems to be "yes" anyway.

Closes #5373

  * runtests.pl:
    - Fix out-of-tree build under CMake when srcdir is not set. Default
      srcdir to the location of runtests.pl.
    - Add a hack to allow CMake to use the TFLAGS option as documented
      in tests/README and used in scripts/travis/script.sh.
  * Bump CMake version to 3.2 for USES_TERMINAL, dropping Debian Jessie
    support (no one should care, it is already EOL.).
  * Remove CTest since it defines its own 'test' target with no tests
    since all unittests are already broken and not built by default.
  * Add new test targets based on the options from Makefile.am. Since
    new test targets are rarely added, I opted for duplicating the
    runtests.pl options as opposed to creating a new Makefile.inc file.
    Use top-level target names (test-x) instead of x-test since that is
    used by CI and others.

Closes #5358

Tested alt-svc with quiche. While at it, add missing MultiSSL reporting
(not tested).

Add three new CMake Find modules (using the curl license, but I grant
others the right to apply the CMake BSD license instead).

This CMake config is simpler than the autotools one because it assumes
ngtcp2 and nghttp3 to be used together. Another difference is that this
CMake config checks whether QUIC is actually supported by the TLS
library (patched OpenSSL or boringssl) since this can be a common
configuration mistake that could result in build errors later.

Unlike autotools, CMake does not warn you that the features are
experimental. The user is supposed to already know that and read the
documentation. It requires a very special build environment anyway.

Tested with ngtcp2+OpenSSL+nghttp3 and quiche+boringssl, both built from
current git master. Use `LD_DEBUG=files src/curl |& grep need` to figure
out which features (libldap-2.4, libssh2) to disable due to conflicts
with boringssl.

Closes #5359

Closes #5173

Fixes #5165
Closes #5167

As there's no code using it.

Closes #5144

My working build cmdline:

$ cmake -DCMAKE_PREFIX_PATH=$HOME/build-wolfssl -DCMAKE_USE_WOLFSSL=ON .

Assisted-by: Brad King
Closes #5095

- Show HTTPS-proxy in the features output for those backends that
  support it: OpenSSL, GnuTLS and NSS.

Prior to this change HTTPS-proxy was missing from the cmake features
output even if curl was built with it. Only cmake output was affected.
Both the library and tool correctly reported the feature.

Bug: https://curl.haxx.se/mail/lib-2020-03/0008.html
Reported-by: David Lopes

Closes https://github.com/curl/curl/pull/5025

- 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