CMake fix

9 jobs for v2.1 in 3 minutes and 13 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #118312
VS2015
VS2015-32-bit

00:01:26

passed #118311
VS2015
VS2015-64-bit

00:01:32

passed #118310
VS2017
VS2017-32-bit

00:01:26

passed #118309
VS2015
VS2017-64-bit

00:01:32

passed #118306
Linux
clang-c++11

00:03:03

passed #118308
Linux
cppcheck

00:03:08

passed #118313
Linux
doc

00:00:05

passed #118305
Linux
gcc-c++11

00:03:12

passed #118307
Apple
macos-c++11

00:01:45