Allow to optionally use C++17

[MarcelKoch]

Based on: #1544

This PR allows to optionally use C++17. If the CMake option CMAKE_CXX_STANDARD is set, this will be the standard that is used to compile ginkgo. If this option is not used, then the highest available standard between c++14, c++17 will be used.

Additionally, the macro GINKGO_HAVE_CXX17 will be set to true or false, depending on the standard.

Exception: DPCPP will still use C++17, and I have no idea of how the HIP setup works, but I'm guessing that it will ignore this.

PR Stack:

• Add segmented array type #1545
• Distributed Index Map #1543
• Adds Index map device kernels #1579
• Use index_map in distributed::matrix #1544
• Allow to optionally use C++17 #1548
• Adds sparse communicator class #1546
• Use sparse communicator in distributed::Matrix #1549

[thoasm]

static_assert should also have an error string (unless it's C++17+):

Suggested change

	int main() { static_assert(__cplusplus >= CXX_STD); }
	int main() { static_assert(__cplusplus >= CXX_STD, "Required C++ standard not supported."); }

[pratikvn]

LGTM! I think we need to note that this will might break the OpenCARP pipelines ? Because they want to explicitly set to C++14. It is not an issue, but probably needs an update from their side to set the GINKGO_CXX_STANDARD.

[pratikvn]

minor nit:

Suggested change

	/* Do we support C++17? */
	/* Does the CXX compiler support C++17? */

[MarcelKoch]

This PR is not required anymore. If we decide at a later point to enable c++17 optionally we can revive this.