Fix configure test for unordered_set.

This had no chance of passing because it was not valid C++.
Factor out the code which tests address sanitizer options into a separate function.
Make the script that tests the sanitizer options take a variable.
Test all compilers for sanitize flags, not just clang.

This is more robust than just assuming GCC >= 4.8 will support it.
Only do the sanitizer tests if they are requested.
Ran bootstrap.