if(DEFINED var) only checks if the var is defined, but with certain Finders
var can be set but NOTFOUND.

For the purposes of the if at hand we want to make sure that we have a
*valid* QT_IMPORTS_DIR not just a set one. To that end if(var) is the
correct instruction here as var being undefined, 0, OFF, IGNORE or NOTFOUND
will end up as a false condition making sure that we only set
MARBLE_QT_IMPORTS_DIR to the value of QT_IMPORTS_DIR iff it is a truthy
value.

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