cmake: Only support building docs the on cmake >= 3.9

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

doc/CMakeLists.txt

@@ -1,6 +1,8 @@
 #
 # Build the documentation
 #
+if (${CMAKE_VERSION} VERSION_GREATER_EQUAL "3.9.0")
+
 find_package(Doxygen)
 
 if (DOXYGEN_FOUND)
@@ -39,4 +41,6 @@ if (DOXYGEN_FOUND)
                      ${CMAKE_SOURCE_DIR}/include/libssh
                      ${CMAKE_SOURCE_DIR}/src
                      ${CMAKE_CURRENT_SOURCE_DIR})
-endif()
+endif() # DOXYGEN_FOUND
+
+endif() # CMAKE_VERSION