doc: tutorial is always installed, doxygen compilation is optional

Makefile.am

@@ -19,10 +19,7 @@ CLEANFILES = *.gcno *.gcda *.linkinfo
 
 SUBDIRS = src
 SUBDIRS += tools tests
-
-if BUILD_DOC
 SUBDIRS += doc
-endif
 
 if USE_MPI
 SUBDIRS += mpi

doc/Makefile.am

@@ -9,7 +9,9 @@
 #
 # See the GNU Free Documentation License in COPYING.GFDL for more details.
 
+if BUILD_DOC
 SUBDIRS = doxygen
+endif
 
 EXTRA_DIST =    tutorial/hello_world.c \
 		tutorial/hello_world_plugin.c \