cmake: fix wrong variable name

9 jobs for v2.1rc1 in 7 minutes and 38 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed #84060
VS2015
VS2015-32-bit

00:01:27

passed #84059
VS2015
VS2015-64-bit

00:01:30

passed #84058
VS2017
VS2017-32-bit

00:01:26

passed #84057
VS2015
VS2017-64-bit

00:01:32

passed #84054
Linux
clang-c++11

00:02:30

passed #84056
Linux
cppcheck

00:05:14

passed #84061
Linux
doc

00:00:09

passed #84053
Linux
gcc-c++11

00:04:43

passed #84055
Apple
macos-c++11

00:02:59