SCons apparently supports accepting unambiguous prefixes of valid options; see e.g. http://scons.org/doc/1.3.1/HTML/scons-api/SCons.compat._scons_optparse.OptionParser-class.html#_match_long_opt
However, if you pass an unambiguous abbreviated option on the command line, scons accepts it but GetOption(name) still returns None when our SConstruct code asks for the fully specified option name.
You can try this with --disable-warnings-as-errors:
$ scons mongod --disable-warnings-as-er
scons either returns an error, or proceeds normally as if --disable-warnings-as-errors had been passed on the command line.
scons proceeds normally as if --disable-warnings-as-errors had NOT been passed on the command line.