Add FindGnuTLS.cmake from 3.17.3.
Addresses #199
diff --git a/cmake-compat/FindGnuTLS.cmake b/cmake-compat/FindGnuTLS.cmake
new file mode 100644
index 0000000..819f000
--- /dev/null
+++ b/cmake-compat/FindGnuTLS.cmake
@@ -0,0 +1,82 @@
+# Distributed under the OSI-approved BSD 3-Clause License. See accompanying
+# file Copyright.txt or https://cmake.org/licensing for details.
+
+#[=======================================================================[.rst:
+FindGnuTLS
+----------
+
+Find the GNU Transport Layer Security library (gnutls)
+
+IMPORTED Targets
+^^^^^^^^^^^^^^^^
+
+This module defines :prop_tgt:`IMPORTED` target ``GnuTLS::GnuTLS``, if
+gnutls has been found.
+
+Result Variables
+^^^^^^^^^^^^^^^^
+
+``GNUTLS_FOUND``
+ System has gnutls
+``GNUTLS_INCLUDE_DIR``
+ The gnutls include directory
+``GNUTLS_LIBRARIES``
+ The libraries needed to use gnutls
+``GNUTLS_DEFINITIONS``
+ Compiler switches required for using gnutls
+``GNUTLS_VERSION``
+ version of gnutls.
+#]=======================================================================]
+
+# Note that this doesn't try to find the gnutls-extra package.
+
+
+if (GNUTLS_INCLUDE_DIR AND GNUTLS_LIBRARY)
+ # in cache already
+ set(gnutls_FIND_QUIETLY TRUE)
+endif ()
+
+if (NOT WIN32)
+ # try using pkg-config to get the directories and then use these values
+ # in the find_path() and find_library() calls
+ # also fills in GNUTLS_DEFINITIONS, although that isn't normally useful
+ find_package(PkgConfig QUIET)
+ PKG_CHECK_MODULES(PC_GNUTLS QUIET gnutls)
+ set(GNUTLS_DEFINITIONS ${PC_GNUTLS_CFLAGS_OTHER})
+ set(GNUTLS_VERSION ${PC_GNUTLS_VERSION})
+ # keep for backward compatibility
+ set(GNUTLS_VERSION_STRING ${PC_GNUTLS_VERSION})
+endif ()
+
+find_path(GNUTLS_INCLUDE_DIR gnutls/gnutls.h
+ HINTS
+ ${PC_GNUTLS_INCLUDEDIR}
+ ${PC_GNUTLS_INCLUDE_DIRS}
+ )
+
+find_library(GNUTLS_LIBRARY NAMES gnutls libgnutls
+ HINTS
+ ${PC_GNUTLS_LIBDIR}
+ ${PC_GNUTLS_LIBRARY_DIRS}
+ )
+
+mark_as_advanced(GNUTLS_INCLUDE_DIR GNUTLS_LIBRARY)
+
+include(${CMAKE_CURRENT_LIST_DIR}/FindPackageHandleStandardArgs.cmake)
+FIND_PACKAGE_HANDLE_STANDARD_ARGS(GnuTLS
+ REQUIRED_VARS GNUTLS_LIBRARY GNUTLS_INCLUDE_DIR
+ VERSION_VAR GNUTLS_VERSION_STRING)
+
+if(GNUTLS_FOUND)
+ set(GNUTLS_LIBRARIES ${GNUTLS_LIBRARY})
+ set(GNUTLS_INCLUDE_DIRS ${GNUTLS_INCLUDE_DIR})
+
+ if(NOT TARGET GnuTLS::GnuTLS)
+ add_library(GnuTLS::GnuTLS UNKNOWN IMPORTED)
+ set_target_properties(GnuTLS::GnuTLS PROPERTIES
+ INTERFACE_INCLUDE_DIRECTORIES "${GNUTLS_INCLUDE_DIRS}"
+ INTERFACE_COMPILE_DEFINITIONS "${GNUTLS_DEFINITIONS}"
+ IMPORTED_LINK_INTERFACE_LANGUAGES "C"
+ IMPORTED_LOCATION "${GNUTLS_LIBRARIES}")
+ endif()
+endif()
