Address a clang compiler problem.

[bangerth]

On @danieldouglas92 's laptop, the Levi-Civita variable introduced by @MFraters yesterday in #5318 leads to a linker error: It complains that there are duplicate copies of the variable in every object file. That's a compiler error: constexpr implies const, and const variables at namespace scope are automatically static, i.e. every .cc file that #includes the header file has its own copy with internal linkage. Apparently, clang does not know that rule.

As verified by @danieldouglas92, the problem can be worked around by explicitly marking the variable as static.

[bangerth]

OK, let's try this again. @MFraters I had to turn this into a function -- I just can't figure out a combination of flags that works for both gcc and @danieldouglas92 's clang.

[bangerth]

Perhaps now? Let's see what the tester says.

[bangerth]

Hallelujah, it finally works!