cmake: do not set cxx standard on indidual projects, to avoid mixing versions

9 jobs for v2.1rc1 in 5 minutes and 8 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #84045
VS2015
VS2015-32-bit

00:01:27

passed #84044
VS2015
VS2015-64-bit

00:01:36

passed #84043
VS2017
VS2017-32-bit

00:01:31

passed #84042
VS2015
VS2017-64-bit

00:01:43

passed #84039
Linux
clang-c++11

00:05:08

passed #84041
Linux
cppcheck

00:03:07

passed #84046
Linux
doc

00:00:12

passed #84038
Linux
gcc-c++11

00:04:56

passed #84040
Apple
macos-c++11

00:02:03