Solution:
This is an issue with the imported Autoconf M4 macro package for standardised
code coverage builds, i.e. using --enable-code-coverage.

The simplest way that I could find is to add a case statement that checks if
the output of running `gcov -version` contains the "LLVM" keyword; if that is
true then do not link with LIBGCOV as its neither required nor supported when
using the GCOV frontend for LLVM; least not on Mac OS X. The case statement
would also be the most portable.

Moreover, using the "-version" argument instead of "-v" seems to be the best
bet as that is supported by the normal GCOV and LLVM GCOV frontend.

Upstream candidate - this solution should be improved by Autoconf M4 macro
overlords and applied to the upstream M4 package; I could not find a suitable
way to detect if LLVM GCOV is being used, except for the solution herein; this
should also work on *BSD too.