require cxx11

[cwsmith]

Tested compilers

• [x] gcc 8.2 (on cooley)
• [x] clang 7 (on aimos)
• [x] intel (19.0.1 on cranium)
• [x] pgi/nvhpc 20.11 (on aimos)
• [x] xl 16 (on aimos)

Documentation update

• [x] https://github.com/SCOREC/core/wiki/General-Build-instructions - no changes needed, we didn't list the CXX11 option

needed for #344

[jacobmerson]

Looks good to me.