Martin Ågren authored Mar 17, 2019 and Junio C Hamano committed Mar 18, 2019

If you build the documentation switching between different options,
e.g., to build with both Asciidoc and Asciidoctor, you'll probably find
yourself running `make -C Documentation clean` either too often (wasting
time) or too rarely (getting mixed builds).

Track the flags we're using in the documentation build, similar to how
the main Makefile tracks CFLAGS and prefix flags. Track ASCIIDOC_COMMON
directly rather than its individual components -- that should make it
harder to forget to update the tracking if/when we modify the build
commands.

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>