Otherwise Clang complains (on all platforms): "variable '<anonymous
namespace>::CompareMatrixElemFunc<<anonymous>::ElemGreaterZero>::maComp'
has internal linkage but is not defined".

I guess this link-time error reported by one tinderbox that uses gcc
is also related: "relocation R_X86_64_PC32 against undefined symbol
`_ZN12_GLOBAL__N_121CompareMatrixElemFuncINS_13ElemEqualZeroEE6maCompE´
can not be used when making a shared object; recompile with
-fPIC". (That mangled name stands for "(anonymous namespace)::
CompareMatrixElemFunc<(anonymous namespace)::ElemEqualZero>::maComp".)

Change-Id: I05a341a55e5f57864a476b5d1614b0e73bc9f017