cmake: add and use local FindGnuTLS module

Replacing a combination of custom logic in the main script and relying
on CMake's built-in Find module, with code and behavior used for
the rest of dependencies.

Also to:
- add version detection in the non-pkg-config path.
- make `GNUTLS_INCLUDE_DIR` and `GNUTLS_LIBRARY` take precedence over
  pkg-config. As with other dependencies.
- document the above two configuration options.
- prepare for #16973, which originally introduced this local Find
  module.

The local module is doing largely the same as CMake's built-in
FindGnuTLS. Differences:
- honors `CURL_USE_PKGCONFIG`.
- returns GnuTLS version for non-pkg-config detection.
- consistently returns `GNUTLS_VERSION`.
  (CMake's built-in uses s different name in <3.16.)
- CMake 3.16+ returns an imported target. curl supports 3.7,
  therefore we may only use it conditionally, which isn't worth it.

Cherry-picked from #16973

Closes #19163

CMake/FindGnuTLS.cmake

@@ -0,0 +1,83 @@
+#***************************************************************************
+#                                  _   _ ____  _
+#  Project                     ___| | | |  _ \| |
+#                             / __| | | | |_) | |
+#                            | (__| |_| |  _ <| |___
+#                             \___|\___/|_| \_\_____|
+#
+# Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+#
+# This software is licensed as described in the file COPYING, which
+# you should have received as part of this distribution. The terms
+# are also available at https://curl.se/docs/copyright.html.
+#
+# You may opt to use, copy, modify, merge, publish, distribute and/or sell
+# copies of the Software, and permit persons to whom the Software is
+# furnished to do so, under the terms of the COPYING file.
+#
+# This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+# KIND, either express or implied.
+#
+# SPDX-License-Identifier: curl
+#
+###########################################################################
+# Find the GnuTLS library
+#
+# Input variables:
+#
+# - `GNUTLS_INCLUDE_DIR`:   The GnuTLS include directory.
+# - `GNUTLS_LIBRARY`:       Path to `gnutls` library.
+#
+# Result variables:
+#
+# - `GNUTLS_FOUND`:         System has GnuTLS.
+# - `GNUTLS_INCLUDE_DIRS`:  The GnuTLS include directories.
+# - `GNUTLS_LIBRARIES`:     The GnuTLS library names.
+# - `GNUTLS_LIBRARY_DIRS`:  The GnuTLS library directories.
+# - `GNUTLS_PC_REQUIRES`:   The GnuTLS pkg-config packages.
+# - `GNUTLS_CFLAGS`:        Required compiler flags.
+# - `GNUTLS_VERSION`:       Version of GnuTLS.
+
+set(GNUTLS_PC_REQUIRES "gnutls")
+
+if(CURL_USE_PKGCONFIG AND
+   NOT DEFINED GNUTLS_INCLUDE_DIR AND
+   NOT DEFINED GNUTLS_LIBRARY)
+  find_package(PkgConfig QUIET)
+  pkg_check_modules(GNUTLS ${GNUTLS_PC_REQUIRES})
+endif()
+
+if(GNUTLS_FOUND)
+  set(GnuTLS_FOUND TRUE)
+  string(REPLACE ";" " " GNUTLS_CFLAGS "${GNUTLS_CFLAGS}")
+  message(STATUS "Found GnuTLS (via pkg-config): ${GNUTLS_INCLUDE_DIRS} (found version \"${GNUTLS_VERSION}\")")
+else()
+  find_path(GNUTLS_INCLUDE_DIR NAMES "gnutls/gnutls.h")
+  find_library(GNUTLS_LIBRARY NAMES "gnutls" "libgnutls")
+
+  unset(GNUTLS_VERSION CACHE)
+  if(GNUTLS_INCLUDE_DIR AND EXISTS "${GNUTLS_INCLUDE_DIR}/gnutls/gnutls.h")
+    set(_version_regex "#[\t ]*define[\t ]+GNUTLS_VERSION[\t ]+\"([^\"]*)\"")
+    file(STRINGS "${GNUTLS_INCLUDE_DIR}/gnutls/gnutls.h" _version_str REGEX "${_version_regex}")
+    string(REGEX REPLACE "${_version_regex}" "\\1" _version_str "${_version_str}")
+    set(GNUTLS_VERSION "${_version_str}")
+    unset(_version_regex)
+    unset(_version_str)
+  endif()
+
+  include(FindPackageHandleStandardArgs)
+  find_package_handle_standard_args(GnuTLS
+    REQUIRED_VARS
+      GNUTLS_INCLUDE_DIR
+      GNUTLS_LIBRARY
+    VERSION_VAR
+      GNUTLS_VERSION
+  )
+
+  if(GNUTLS_FOUND)
+    set(GNUTLS_INCLUDE_DIRS ${GNUTLS_INCLUDE_DIR})
+    set(GNUTLS_LIBRARIES    ${GNUTLS_LIBRARY})
+  endif()
+
+  mark_as_advanced(GNUTLS_INCLUDE_DIR GNUTLS_LIBRARY)
+endif()