modesetting: Fix broken manpage in autoconf build
The autoconf build for the modesetting driver still relied on xorg-macros.m4 for string replacements and did not include the top-level manpages.am. As a result, no substitutions took place after commit 2e497bf8.
This should be a candidate for the 1.20 branch.
On master there is also a follow-up regression test (commit a505ecba), but I did not dare backporting it to 1.20, because the
.gitlab-ci.yml is very different there.