Don't try to install doxygen related stuff, if doxygen is not installed.

This fixes #803, but leaves us with the ugly situation, that docs in the 
tar balls don't get installed, if doxygen is missing.

[[Imported from SVN: r6435]]

am/doxygen

@@ -28,6 +28,8 @@ endif # DUNEWEB
 
 ####
 # how to build doxygen documentation
+if DOXYGEN
+
 if BUILD_DOCS
   DOXYGENTAG = doxygen-tag
   # EXTRAINSTALL in the html subdirectory
@@ -35,8 +37,6 @@ if BUILD_DOCS
   DOXYGENINSTALL = $(DOXYGENHTMLFILES) ../doxygen.log ../doxyerr.log
 endif # BUILD_DOCS
 
-if DOXYGEN
-
 DUNEDOXYNIZE=@DUNE_COMMON_ROOT@/bin/dunedoxynize
 $(srcdir)/Doxyfile.in: FORCE
 	if test -f $(srcdir)/Doxylocal && test $(srcdir)/Doxylocal -nt $(srcdir)/Doxyfile.in; then \