Omit fcheck-new for clang on OSX

Like the comment says, strictly speaking one should check expressly
for clang instead of just Darwin, but I bet that is the most common
place where clang is used.
Allow compiler warnings because of clang

Clang has some *serious* opinions about warnings in code, and thus
having `-Werror` is going to ruin your day.
Merge pull request #7 from mdaniel/clang-5

Be sensitive to the number of warnings-as-errors emitted by clang 5