This was previously defined by the obsolete AC_TYPE_SIGNAL macro,
which was removed in 2682e5f5. The deprecation text says

> Your code may safely assume C89 semantics that RETSIGTYPE is void.

So, remove it and just use void instead.

Closes #6861

- `HAVE_STRNCASECMP`
- `HAVE_TCGETATTR`
- `HAVE_TCSETATTR`

Reviewed-by: Jay Satiro
Reviewed-by: Daniel Stenberg
Closes #6645

By differentiating between ON and AUTO it can make a missing zlib
library a hard error when CURL_ZLIB=ON is used.

Reviewed-by: Jakub Zakrzewski
Closes #6221
Fixes #6173

When using clang-cl on windows -fvisibility=hidden is not an known
argument. Instead it behaves exactly like MSVC in this case. So let's
make sure we take that path.

In CMake clang-cl sets both CMAKE_C_COMPILER_ID=clang and MSVC get's
defined since clang-cl is basically a MSVC emulator. So guarding like we
do in this patch seems logical.

Reviewed-by: Jakub Zakrzewski
Closes #6194

Follow-up from 4d2f8006777

Closes #6172

include zstd curl patch for Makefile.m32 from vszakats
and include Add CMake support for zstd from Peter Wu

Helped-by: Viktor Szakats
Helped-by: Peter Wu
Closes #5453

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

We use `check_c_source_compiles` to check possible send/recv signatures
by reproducing the forward declarations from system headers.  On Windows
the `winsock2.h` header adds dll linkage settings to its forward
declaration.  If ours does not match the compiler warns:

    warning C4273: 'recv': inconsistent dll linkage

Add `WINSOCK_API_LINKAGE` to our test signatures when it is defined so
that our linkage is consistent with that from `winsock2.h`.

Fixes #4764
Closes #5232

Reported by the new script 'scripts/copyright.pl'. The script has a
regex whitelist for the files that don't need copyright headers.

Removed three (mostly usesless) README files from docs/

Closes #5141