Fix test -nt for NetBSD and closes #168.

Author: gijsbers

doc/Makefile.am

@@ -13,11 +13,13 @@ adoc_input = $(srcdir)/icewm.adoc
 
 $(html_documentation):
 	$(AM_V_GEN)if test -x "`which $(MARKDOWN) 2>/dev/null`"; then \
-		if test $(mark_input) -nt $(html_documentation); then \
+		if test $(mark_input) -nt $(html_documentation) \
+			-o ! -s $(html_documentation); then \
 			$(MARKDOWN) $(mark_input) >$@; \
 		fi; \
 	elif test -x "`which $(ASCIIDOC) 2>/dev/null`"; then \
-		if test $(adoc_input) -nt $(html_documentation); then \
+		if test $(adoc_input) -nt $(html_documentation) \
+			-o ! -s $(html_documentation); then \
 			$(ASCIIDOC) $(ASCIIDOC_BACKEND) -o $@ $(adoc_input); \
 		fi; \
 	else \