SERVER-13686 Do not change the variant directory based on the JS engi…

…ne being compiled.

Having the variant directory contain the JS engine name can actually increase
the time required to switch JS engines, because the only code that must compile
when you switch is the new JS engine and a small handful of support files.

SConstruct

@@ -231,7 +231,7 @@ add_option( "ssl" , "Enable SSL" , 0 , True )
 
 # library choices
 js_engine_choices = ['v8-3.12', 'v8-3.25', 'none']
-add_option( "js-engine", "JavaScript scripting engine implementation", 1, True,
+add_option( "js-engine", "JavaScript scripting engine implementation", 1, False,
            type='choice', default=js_engine_choices[0], choices=js_engine_choices)
 add_option( "libc++", "use libc++ (experimental, requires clang)", 0, True )