Variable: installation-directory

A directory within which to look for the `lib-src' and `etc' directories.
In an installed Emacs, this is normally nil. It is non-nil if
both `lib-src' (on MS-DOS, `info') and `etc' directories are found
within the variable `invocation-directory' or its parent. For example,
this is the case when running an uninstalled Emacs executable from its
build directory.