configure.ac: fail it doxygen not found
configure.ac: fail it doxygen not found

This change allows improving the AM_CONDITIONAL call.

sl0th/libmpdclient,jwise/libmpdclient,yusiwen/libmpdclient,sl0th/libmpdclient,jwise/libmpdclient,jwise/libmpdclient,sl0th/libmpdclient,yusiwen/libmpdclient,yusiwen/libmpdclient