Tarball build fails with "fatal: not a git repository (or any parent up to mount point /)"

Build from latest tarball:

wget https://ftp.gnu.org/gnu/libidn/libidn2-2.0.5.tar.gz
tar xvf libidn2-2.0.5.tar.gz
cd libidn2-2.0.5/
./configure
make

stops with

make[2]: Leaving directory '/tmp/libidn2-2.0.5/po'
make[2]: Entering directory '/tmp/libidn2-2.0.5'
fatal: not a git repository (or any parent up to mount point /)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).
fatal: not a git repository (or any parent up to mount point /)
Stopping at filesystem boundary (GIT_DISCOVERY_ACROSS_FILESYSTEM not set).