When using project files, library installation is part of the library build
process. Thus no further action is needed in order to make use of the
libraries that are built as part of the general application build. A usable
version of the library is installed in the directory specified by the
Library_Dir attribute of the library project file.
You may want to install a library in a context different from where the library
is built. This situation arises with third party suppliers, who may want
to distribute a library in binary form where the user is not expected to be
able to recompile the library. The simplest option in this case is to provide
a project file slightly different from the one used to build the library, by
using the externally_built attribute. Using Library Projects