fixed bug in cmake that prevented proper detection of qt < 5.6

Status Job ID Name Coverage
  Test
passed #5361
MultiThreads Linux
clang-c++11-Qt4

00:06:57

passed #5365
MultiThreads Linux
clang-c++11-Qt5

00:08:44

passed #5363
MultiThreads Linux
clang-c++98-Qt4

00:06:14

passed #5367
MultiThreads Linux
clang-c++98-Qt5

00:07:32

passed #5360
Linux
gcc-c++11-Qt4

00:03:25

passed #5364
Linux
gcc-c++11-Qt5

00:04:05

passed #5362
Linux
gcc-c++98-Qt4

00:03:47

passed #5366
Linux
gcc-c++98-Qt5

00:03:17