Most third party code in HTCondor is managed via "externals"
These are either wget'd/untarred/build into the bld_externals
directory before HTCondor proper is built, or, assumed to
be installed by the native OS package manager.

However, some third party code, notably, header-only C++
code, is checked in directory into the HTCondor source
tree. This method, known as "vendoring" may be used
for packages which don't have native packages, or are
small, or whose version we want to more directly control.

