Add a new "coverage" Makefile target that re-builds the
library and tests using the gcc compiler/linker flags
required by gcov, runs the test suite to capture the
runtime data, then compiles a coverage report.

The report, which is saved in a file named "untested",
consists of a list of untested files, followed by a list
of untested functions. More detailed execution statistics
are given in the gcov log files which are saved in the
top-level directory (named like src#hash.c.gcov).

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>