cmake: detect TLS-SRP in OpenSSL/wolfSSL/GnuTLS

With new option `CURL_DISABLE_SRP=ON` to force-disable it.
To match existing option and detection logic in autotools.

Also:
- fix detecting GnuTLS.
  We assume `nettle` as a GnuTLS dependency.
- add CMake GnuTLS CI job.
- bump AppVeyor CMake OpenSSL MSVC job to OpenSSL 1.1.1 (from 1.0.2)
  TLS-SRP fails to detect with 1.0.2 due to an OpenSSL header bug.
- fix compiler warning when building with GnuTLS and disabled TLS-SRP.
- fix comment typos, whitespace.

Ref: #11964

Closes #11967

.github/workflows/macos.yml

@@ -212,6 +212,9 @@ jobs:
         - name: libssh2
           install: nghttp2 openssl libssh2
           generate: -DOPENSSL_ROOT_DIR=/usr/local/opt/openssl -DCURL_USE_LIBSSH2=ON -DBUILD_SHARED_LIBS=ON -DBUILD_STATIC_LIBS=ON
+        - name: GnuTLS
+          install: gnutls
+          generate: -DCURL_USE_GNUTLS=ON -DCURL_USE_OPENSSL=OFF -DCURL_DISABLE_LDAP=ON -DCURL_DISABLE_LDAPS=ON -DCMAKE_SHARED_LINKER_FLAGS=-L/usr/local/lib -DCMAKE_EXE_LINKER_FLAGS=-L/usr/local/lib
     steps:
     - run: echo libtool autoconf automake pkg-config ${{ matrix.build.install }} | xargs -Ix -n1 echo brew '"x"' > /tmp/Brewfile
       name: 'brew bundle'