Upstream uses a specific cmake option/variable (`MANDIR`[1]) for
configuring the man pages install directory.

Note: they do not use the standard cmake option/variable
      `CMAKE_INSTALL_MANDIR`.

[1] https://github.com/liballeg/allegro5/blob/5.2.10.1/docs/Refman.cmake#L333

This conforms to the 'CMake Package Guidelines'[1]
in the Arch Wiki.

[1] https://wiki.archlinux.org/title/CMake_package_guidelines

FTR, commands used to change arch=() and then verify the changes:

$ sed -Ei "/^arch=/{s/ *?['\"]?i686['\"]? *?//;s/ *$//}" */*/PKGBUILD*

$ grep -r i686 * | grep arch= | grep -v repos

$ svn diff | grep -A 1 ^-[^-] | grep -v -- -- | cut -c2- |
  while IFS=$'\n' read -r l1; IFS=$'\n' read -r l2; do
    echo "$l1 -> $l2"
  done | sort -u