builds fail outside of git tree in CentOS.

Reproduced in 18.10:

Steps to reproduce:

git checkout v18.10
cd vpp
make install-dep
make dist

...in a clean directory
xz -dc /home/therbert/vpp/build-root/vpp-18.10-release.tar.xz | /usr/bin/tar -xf -
cd vpp-18.10
make build-release

Output is attached.