Makefile: build docs only for enabled tools; fix docs dependencies

This makes "make documentation" build the man/html pages only for
the tools enabled in the build.  It also fixes the dependency
tracking for the built man pages.
Signed-off-by: Mans Rullgard <mans@mansr.com>
(cherry picked from commit 7f939f55)

Makefile

@@ -112,7 +112,7 @@ version.h .version:
 
 alltools: $(TOOLS)
 
-DOCS = $(addprefix doc/, developer.html faq.html general.html libavfilter.html $(ALLHTMLPAGES) $(ALLMANPAGES))
+DOCS = $(addprefix doc/, developer.html faq.html general.html libavfilter.html) $(HTMLPAGES) $(MANPAGES) $(PODPAGES)
 
 documentation: $(DOCS)