Allow turning off boost erf (which crashes valgrind) (PLAT-568)

This allows you to compile with UNDER_VALGRIND=1 defined, which will avoid
the problematic code (note that it also stops IRLS from working, so it's
only a small workaround).

Has no impact unless enabled.
Merge pull request #7 from datacratic/PLAT-568-valgrind-crashing-boost-erf

Allow turning off boost erf (which crashes valgrind) (PLAT-568)