Add cxx_std_11 CMake compile feature to PCU

[bobpaw]

Add cxx_std_11 feature to PCU

std::unique_ptr was introduced in C++11 and is used by the new public PCU interface.

When the C++ standard is not set with CMAKE_CXX_STANDARD and does not default to at least C++11 for the selected compiler, errors are emitted when other projects try to link against PUMI. The default build behavior should be successful.

This change allows the C++11 requirement to propagate to targets that link against pcu. The default PUMI C++ standard is already C++11:

core/CMakeLists.txt

Lines 28 to 37 in 395d3a2

	if(NOT ENABLE_CGNS AND NOT ENABLE_OMEGA_H)
	message(STATUS "enabling cxx11")
	bob_set_cxx_standard(11)
	elseif(ENABLE_CGNS)
	message(STATUS "enabling cxx14")
	bob_set_cxx_standard(14)
	elseif(ENABLE_OMEGA_H)
	message(STATUS "enabling cxx17")
	bob_set_cxx_standard(17)
	endif()

This change propagates that setting outward for at least the pcu library (which the core links against, and so the library as a whole gets cxx_std_11 as a PUBLIC feature).

[bobpaw]

@cwsmith, we ran into errors when building a project that links against PUMI with a compiler that didn't default to C++11. We were using -DCMAKE_CXX_STANDARD=11 as a workaround but I wanted to get this fix into the upstream so that it isn't required.

[cwsmith]

/runtests

[github-actions]

Build Log
Simmetrix Test Result: success
Simmetrix + CGNS Test Result: success

[cwsmith]

Thank you.