Fix build variable botan_pkgconfig
Fix build variable botan_pkgconfig

This avoids an extra "hello/" path segment when configured with
`./configure.py --with-build-dir hello`

webmaster128/botan,Rohde-Schwarz-Cybersecurity/botan,Rohde-Schwarz-Cybersecurity/botan,webmaster128/botan,Rohde-Schwarz-Cybersecurity/botan,randombit/botan,Rohde-Schwarz-Cybersecurity/botan,randombit/botan,randombit/botan,webmaster128/botan,randombit/botan,webmaster128/botan,webmaster128/botan,Rohde-Schwarz-Cybersecurity/botan,Rohde-Schwarz-Cybersecurity/botan,randombit/botan