Building libcapsule from git requires GNU autoconf-archive macros. The
oldest supported version is 20180313-1 from Debian 9 'stretch'. If your
development system or container is older, unpack or clone a suitable
version of autoconf-archive and use commands like:

   libcapsule$ ACLOCAL_PATH=/path/to/autoconf-archive/m4 NOCONFIGURE=1 ./autogen.sh
   libcapsule$ ./configure
   libcapsule$ make

Building from tarball releases removes this requirement.
