root7 is turned off by default even if the default C++ standard of the compiler is C++14 or above

[eguiraud]

Describe the bug

With ROOT-10692 fixed, ROOT now, by default, uses the default C++ standard of the compiler rather than always using C++11. However, due to how our cmake logic is structured, root7 is still turned off by default, even if the default C++ standard used by the compiler was detected to be C++14 or higher.

Expected behavior

With a compiler that defaults to -std=C++14 or above, a vanilla cmake path/to/root should have root7 turned on.

Additional context

I think the root cause is that, at the following lines in our main CMakeLists.txt, we first include RootBuildOptions (which sets root7 to OFF by default because it doesn't detect a high-enough C++ standard) and then we include CheckCompiler, which sets our default CMAKE_CXX_STANDARD to the compiler default.

root/CMakeLists.txt

Lines 128 to 134 in 33458dc

	#---Load some basic macros which are needed later for the confiuration and build----------------
	include(SearchRootCoreDeps)
	include(RootBuildOptions)
	include(RootMacros)
	include(CheckCompiler)
	include(CheckAssembler)
	include(CheckIntrinsics)

Moving include(CheckCompiler) above include(RootBuildOptions) fixes this issue but breaks Windows, because some cmake variable that CheckCompiler needs in the case of windows were defined earlier.

[eguiraud]

Looking into it a bit more, I think the right fix is indeed to move CheckCompiler above RootBuildOptions, and then fix the problem with Windows.

That problem is a circular dependency:

1. RootBuildOptions.cmake depends on CheckCompiler.cmake, because e.g. the default value of the root7 option (set in RootBuildOptions) depends on the value of CMAKE_CXX_STANDARD (set in CheckCompiler)
2. however, CheckCompiler.cmake includes SetUpWindows.cmake, which uses CMAKE_INSTALL_INCLUDEDIR in a couple of install commands. CMAKE_INSTALL_INCLUDEDIR is set by RootInstallDirs which is included by RootBuildOptions, completing the circle

@amadio suggested moving the install commands Windows needs from SetUpWindows to RootConfiguration.