Fixes to the 'make cover' targets

[Nick Wellnhofer]

Me and Matt have been unable to generate coverage reports, so I had a 
look at the make cover targets. The actual problem were changes and a 
regression in recent Devel::Cover versions, but there turned out to be 
some more problems.

See branch nwellnhof/make_cover

Changes:

* Run gcov from build dir to make it find and include the source code

Currently, the .gcov files don't contain the source code because gcov 
isn't run from the build directory.

* Make HTML report generated with Devel::Cover optional

This makes it possible to generate only the .gcov files for users 
without Devel::Cover.

* Work around Debian bug #535755

* Pass -no-gcov to cover, we run gcov and gcov2perl ourselves

Currently, cover runs gcov and gcov2perl a second time, which is 
unnecessary and fails with recent Devel::Cover versions.

* Set default database for quickcover to cover_db

We shouldn't generate the report directly in the build dir.

* Make quickcover run 'make test_core' to improve coverage

Improves coverage of quickcover at little extra cost.

* Use the same source directories for quickcover

Includes more source files at little extra cost.


Testers are welcome.

Nick