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 clang-c++11 #84054
Linux

00:02:30

passed cppcheck #84056
Linux

00:05:14

passed doc #84061
Linux

00:00:09

passed gcc-c++11 #84053
Linux

00:04:43

passed macos-c++11 #84055
Apple

00:02:59

passed VS2015-32-bit #84060
VS2015

00:01:27

passed VS2015-64-bit #84059
VS2015

00:01:30

passed VS2017-32-bit #84058
VS2017

00:01:26

passed VS2017-64-bit #84057
VS2015

00:01:32