tools/Makefile.am: doc/Makefile.am: add missing showcheck rules

doc/Makefile.am

@@ -61,3 +61,6 @@ uninstall-local:
 # Rule to update documentation on web server. Should only be used locally.
 update-web:
 	scp starpu.pdf starpu.html sync:/web/runtime/html/StarPU
+
+showcheck:
+	-cat /dev/null

tools/Makefile.am

@@ -78,3 +78,5 @@ dist_man1_MANS =\
 	starpu_workers_activity.1
 endif
 
+showcheck:
+	-cat /dev/null