ctest: fix find_program usage
ctest: fix find_program usage

if the variable passed to find_program is set to an empty string, then
find_program will do nothing. It has to be unset or VAR-NOTFOUND for
find_program to work. Modified the code accordingly.

Signed-off-by: Matthias Kretz <372ea4eca737a13cd010ee806d4ce7b98ee71550@kde.org>

chr-engwer/Vc,chr-engwer/Vc,VcDevel/Vc,chr-engwer/Vc,VcDevel/Vc,VcDevel/Vc,VcDevel/Vc