Determine C++ version at library configure time, not at client build time.

9 jobs for cmake-overhaul-3 in 3 minutes and 57 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #81198
VS2015
VS2015-32-bit

00:01:17

passed #81197
VS2015
VS2015-64-bit

00:01:32

passed #81196
VS2017
VS2017-32-bit

00:01:22

passed #81195
VS2015
VS2017-64-bit

00:01:32

passed #81192
Linux
clang-c++11

00:03:56

passed #81194
Linux
cppcheck

00:02:44

passed #81199
Linux
doc

00:00:22

passed #81191
Linux
gcc-c++11

00:03:07

passed #81193
Apple
macos-c++11

00:02:34