Allow scons to pick up directory paths for where to put binaries, header files, shared and static libraries and man pages from the command line.
This is basically the same as whats supported by autoconf/automake and friends. Typically you are able to alter the locations where binaries, header files, libraries etc are put when running ./configure making is super simple to package mongodb in different Linux distributions packaging systems.
Defaults if these options are left out could be the current layout (backwards compatible) or a layout based in /usr/local like most automake/autoconf based builds.
The current --prefix can be left as is and its value, if specified, will be prepended to to values of the "new" arguments making it possible to both specify a certain layout and the move that whole layout around (typically this is used when building packages for Linux distributions).