Fix CMake CXX_STANDARD handling

Fix CMake CXX_STANDARD handling

https://sigrok.org/gitweb/?p=pulseview.git;a=commit;f=CMakeLists.txt;h=fa8d0fcb4cff4f19943fe3ff152dc1428b400b01 introduced a CXX_STANDARD value of 17 while still only requiring cmake 2.8.12.

However, https://cmake.org/cmake/help/latest/prop_tgt/CXX_STANDARD.html#cxx-standard shows that a value of 17 was introduced with cmake 3.8, leading to

"CXX_STANDARD is set to invalid value '17'" when trying to build with cmake < 3.8.

Hence, for versions below 3.8, we only check for up to 14.