do not force the use of static boost libraries if the ENABLE_SHARED variable is undefined

we only want to force the use of shared libraries if ENABLE_SHARED is
set to true. if it is unset, we don't care.

note that this makes ENABLE_SHARED a tristate variable:

 - "ON" -> force shared libs
 - "OFF" -> force static libs
 - <undefined> -> do not care

maybe the third behavior is also desired for the second case, but I don't really know...