Bump Doxygen to support HTML_EXTRA_STYLESHEET

This is the minimum version required - happy to bump it higher if
preferred

Author: tomchadwin

doc/CMakeLists.txt

@@ -28,7 +28,7 @@ INSTALL(FILES ${QGIS_DOC_FILES} DESTINATION ${QGIS_DATA_DIR}/doc)
 INSTALL(FILES ../images/icons/qgis-icon-60x60.png DESTINATION ${QGIS_DATA_DIR}/doc/images)
 
 IF(WITH_APIDOC)
-  FIND_PACKAGE(Doxygen)
+  FIND_PACKAGE(Doxygen 1.8.2 REQUIRED)
   IF(DOXYGEN_FOUND)
 
     FIND_FILE(QT_TAG_FILE