The build process can be controlled via environment variables
similar to other bundled libraries in borgbackup. The main difference
is probably that upstream does not provide a pkgconfig file for
xxhash.
Therefore borg will probably fail to detect the system-provided
version by default (tested on Fedora, seems like Debian and Ubuntu
do not ship a pkgconfig file either). I kept the pkgconfig lookup
code anyway to keep the code as similar as possible to
"setup_compress.py"/"setup_crypto.py".
Setting BORG_LIBXXHASH_PREFIX=/usr helps borg to detect xxhash
on my system (Fedora). You can force the use of the bundled
version of xxhash by setting BORG_USE_BUNDLED_XXHASH=1.
