Guard Doxygen finder to reduce configure times

cmake/ACGDoxygen.cmake

@@ -18,8 +18,9 @@
 # author Jan Woetzel 2004-2006
 # www.mip.informatik.uni-kiel.de/~jw
 
-
-FIND_PACKAGE(Doxygen)
+if ( NOT DOXYGEN_FOUND)
+  FIND_PACKAGE(Doxygen)
+endif()
 
 IF (DOXYGEN_FOUND)
 
@@ -119,4 +120,4 @@ IF (DOXYGEN_FOUND)
     ENDIF(HTML_HELP_COMPILER)
     # MESSAGE(SEND_ERROR "HTML_HELP_COMPILER=${HTML_HELP_COMPILER}")
   ENDIF (WIN32) 
-ENDIF(DOXYGEN_FOUND)
\ No newline at end of file
+ENDIF(DOXYGEN_FOUND)