./configure --enable-gcov adds the required flags to link everything ready for
gcov. A new make gcov target runs the test suite, then pulls all the gcov bits
together into ./test/gcov-reports/ including a summary file.

The script to pull everything out is used in libevdev too, we just have an
extra condition here to ignore the selftest gcov bits (it overwrites the
useful litest.c coverage output).

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>