fix: Remove OS X 10.6 gcov workaround in C++ implementation of Subprocess::popen

This workaround should no longer be needed now that the most recent OS X version is 10.10.
#393 Fix Doxygen generated API documentation of BASIS

With the default setting BASIS_USE_TARGET_UIDS=OFF, the target name of the default Doxygen filter is "doxyfile" without the leading "basis." namespace identifier.