Speed up man page generation

by a factor of 3
Without this patch, using make -j
built every man page and html page 25 times

Now, man pages are always generated in a single ronn call
after all .ronn files are made.

Workarounds GNU make bug https://savannah.gnu.org/bugs/?54854

Makefile

@@ -58,8 +58,10 @@ man-pages: $(HELP_ALL:=.ronn) $(HELP_ALL) $(HELP_ALL:=.txt)
 %.txt: %.ronn
 	groff -Wall -mtty-char -mandoc -Tutf8 -rLL=$(TEXT_WIDTH)n $< | col -b >$@
 
-%.1: %.1.ronn bin/ronn
+$(HELP_ALL): share/man/man-pages.stamp
+share/man/man-pages.stamp: bin/ronn $(HELP_ALL:=.ronn)
 	bin/ronn --organization=GITHUB --manual="Hub Manual" share/man/man1/*.ronn
+	touch $@
 
 %.1.ronn: bin/hub
 	bin/hub help $(*F) --plain-text | script/format-ronn $(*F) $@