Make fontconfig use directory contents rather than modification time to
determine cache validity (by pretending that mtime is broken).
* gnu/packages/patches/fontconfig-cache-ignore-mtime.patch: New file.
* gnu/local.mk (dist_patch_DATA): Register it.
* gnu/packages/fontutils.scm (fontconfig)[source]: Use it.
[arguments]: Unset SOURCE_DATE_EPOCH for tests.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>
