Track coverage of C++ code, too

[fingolfin]

It would be good to track coverage of the C++ code in deps/src, too (not just of the Julia code in src). This probably involves convincing the cmake build system to insert --coverage into CXXFLAGS and LDFLAGS used for building libsingularwrap.

[fingolfin]

Now that libsingular sources are back in, we could try to do this. Some thoughts:

• we can touch deps/src/COVERAGE or something like that to force the treehash for deps/src to differ, and thus for the code there being used instead of the JLL
• next we need a way to add --coverage to CXXFLAGS and LDFLAGS
• it may be necessary to invoke gcov or something like that after the test suite did run