it must be possible configure/disable modules from the outside, so environment-specific adjustments can be conveniently done by the user. this is particularly important for probing modules, see QBS-24.

suggested syntax:

# tip off the probing module where to look for the library:
qbs build modules.openssl.prefix:/usr/local/openssl-1.0.0g

this mechanism could be used for disabling optional dependencies which are actually satisfied, too:

qbs build modules.openssl.enabled:false

and to take it further, one could disable specific dependencies in specific products only:

qbs build products.hello.modules.openssl.enabled:false

the simpler constructs would be good targets for the more familiar configure-like option style, see QBS-266.