Set c++ standard to 17 (#376)

* Set CPP standard to 17

* merge with existing cmake call

Author: Barthelemy

CMakeLists.txt

@@ -69,6 +69,7 @@ unset(isSystemDir)
 # C++ standard
 if(NOT DEFINED CMAKE_CXX_STANDARD)
   set(CMAKE_CXX_STANDARD 17)
+  set(CMAKE_CXX_STANDARD_REQUIRED TRUE)
 endif()
 
 # Add compiler flags for warnings and (more importantly) fPIC and debug symbols