doc/Makefile: install .3 man pages to man3 subdir

Fix trac ticket #2585.

doc/Makefile

@@ -14,7 +14,9 @@ COMPONENTS-$(CONFIG_AVFORMAT)   += ffmpeg-formats ffmpeg-protocols
 COMPONENTS-$(CONFIG_AVDEVICE)   += ffmpeg-devices
 COMPONENTS-$(CONFIG_AVFILTER)   += ffmpeg-filters
 
-MANPAGES    = $(PROGS-yes:%=doc/%.1)    $(PROGS-yes:%=doc/%-all.1)    $(COMPONENTS-yes:%=doc/%.1)    $(LIBRARIES-yes:%=doc/%.3)
+MANPAGES1   = $(PROGS-yes:%=doc/%.1)    $(PROGS-yes:%=doc/%-all.1)    $(COMPONENTS-yes:%=doc/%.1)
+MANPAGES3   = $(LIBRARIES-yes:%=doc/%.3)
+MANPAGES    = $(MANPAGES1) $(MANPAGES3)
 PODPAGES    = $(PROGS-yes:%=doc/%.pod)  $(PROGS-yes:%=doc/%-all.pod)  $(COMPONENTS-yes:%=doc/%.pod)  $(LIBRARIES-yes:%=doc/%.pod)
 HTMLPAGES   = $(PROGS-yes:%=doc/%.html) $(PROGS-yes:%=doc/%-all.html) $(COMPONENTS-yes:%=doc/%.html) $(LIBRARIES-yes:%=doc/%.html) \
               doc/developer.html                                        \
@@ -93,13 +95,16 @@ install-progs-$(CONFIG_DOC): install-man
 
 install-man: $(MANPAGES)
 	$(Q)mkdir -p "$(MANDIR)/man1"
-	$(INSTALL) -m 644 $(MANPAGES) "$(MANDIR)/man1"
+	$(INSTALL) -m 644 $(MANPAGES1) "$(MANDIR)/man1"
+	$(Q)mkdir -p "$(MANDIR)/man3"
+	$(INSTALL) -m 644 $(MANPAGES3) "$(MANDIR)/man3"
 endif
 
 uninstall: uninstall-man
 
 uninstall-man:
-	$(RM) $(addprefix "$(MANDIR)/man1/",$(MANPAGES))
+	$(RM) $(addprefix "$(MANDIR)/man1/",$(MANPAGES1))
+	$(RM) $(addprefix "$(MANDIR)/man3/",$(MANPAGES3))
 
 clean:: docclean