Allow dependent libraries to use lower standards as FairRoot

[YanzhaoW]

Is your feature request related to a problem? Please describe.

In the cmake/modules/CheckCompiler.cmake file, the macro forces the dependent library (e.g. R3BRoot) to use the standard from FairRoot:

FairRoot/cmake/modules/CheckCompiler.cmake

Lines 48 to 53 in 5863027

	if (FairRoot_CXX_STANDARD)
	if ((NOT CMAKE_CXX_STANDARD)
	OR (CMAKE_CXX_STANDARD VERSION_LESS FairRoot_CXX_STANDARD))
	set(CMAKE_CXX_STANDARD "${FairRoot_CXX_STANDARD}")
	endif()
	endif()

This makes the dependent libraries very inflexible as sometimes user needs to compile with a lower standard to check backward compatibility.

Describe the solution you'd like

Either remove these lines or use something like FairRoot_CXX_MIN_STANDARD instead of FairRoot_CXX_STANDARD .

[dennisklein]

There is certainly room to relax this. That said, note, with portions of the dependency tree that shall be used with ROOT cling, we have made bad experiences in the past. The way ROOT cling maps symbols into modules was very fragile when combining different std levels a few years ago (the macro compiler was crashing even). I have not tried it recently though and don't know if the situation improved.

[YanzhaoW]

I see. Maybe we could have an argument for the MACRO. If this variable is false (or true), we don't force the change of CMAKE_CXX_STANDARD.