cmake: Use FindDoxygen package provided by cmake

Signed-off-by: Andreas Schneider <asn@cryptomilk.org>

CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.2.0)
+cmake_minimum_required(VERSION 3.3.0)
 cmake_policy(SET CMP0048 NEW)
 
 project(libssh VERSION 0.8.0 LANGUAGES C)

doc/CMakeLists.txt

@@ -1,5 +1,9 @@
 #
 # Build the documentation
 #
-include(UseDoxygen OPTIONAL)
+find_package(Doxygen)
 
+if (DOXYGEN_FOUND)
+    doxygen_add_docs(docs
+                     ${CMAKE_CURRENT_SOURCE_DIR})
+endif()