CMake fix

9 jobs for v2.1rc2 in 4 minutes and 41 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed clang-c++11 #117778
Linux

00:04:41

passed cppcheck #117780
Linux

00:03:04

passed doc #117785
Linux

00:00:05

passed gcc-c++11 #117777
Linux

00:04:23

passed macos-c++11 #117779
Apple

00:02:57

passed VS2015-32-bit #117784
VS2015

00:01:22

passed VS2015-64-bit #117783
VS2015

00:01:33

passed VS2017-32-bit #117782
VS2017

00:01:25

passed VS2017-64-bit #117781
VS2015

00:01:31