Recently tried to build a PKGBUILD (iceweasel-hardened-preferences) with whirlpoolsums in it.

libremakepkg threw the following error:

==> Starting pre-build activities...
 |  ==> Downloading blacklist of proprietary software packages...done
 |    -> Inspecting package pkgname=iceweasel-hardened-preferences (1.0-2)
==> Downloading sources...
 |  ==> ERROR: Cannot find the whirlpoolsum binary required for source file checksums operations.
 |  ==> ERROR: Could not download sources.

pacman -Fo whirlpoolsum returns no results
sym linking /usr/bin/whirlpoolsum -> rhash --whirlpool (in the chroot) did not help
as it is clearly looking to download a package containing whirlpoolsum

removing those sums from the PKGBUILD (they seems to be some sort of duplicate or backup sums)
Cleared the error and allowed the build to go ahead