Makefile: avoid multiple patterns when recipes generate one file

Makefile: avoid multiple patterns when recipes generate one file

A GNU make pattern rule with multiple targets has always meant that
a single invocation of the recipe will build all the targets.
However in older versions of GNU make a recipe that did not really
build all the targets would be tolerated.

Starting with GNU make 4.4 this behavior is deprecated and pattern
rules are expected to generate files to match all the patterns.
If not all targets are created then GNU make will not consider any
target up to date and will re-run the recipe when it is run again.

Modify Documentation/Makefile to split the man page-creating pattern
rule into a separate pattern rule for each pattern.

Reported-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Paul Smith <psmith@gnu.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>