Also perform coverage analysis with LCOV on OSX

[tovrstra]

Currently we only use gcov on Linux, but clang has a similar interface to produce gcov-compatible coverage output.

This is the doc for clang: https://clang.llvm.org/docs/SourceBasedCodeCoverage.html

We should add compiler flags -fprofile-instr-generate -fcoverage-mapping for a DEBUG build with the clang compiler. clang can produce gcov-compatible outputs, which means we have to pass an alternative gcov binary to the gcovr program in Roberto.