configure.ac: enable subdir-objects

configure.ac: enable subdir-objects

The main thing holding us back here was our gcov hacks. We used to rebuild the
libevdev sources locally inside test/ with the gcov flags so that we could
leave the main libevdev sources untouched. This doesn't work well with
subdir-objects - we have to link to libevdev.la instead.

To enable gcov, we now have to apply the gcov flags to the main library
object. But this also means that when running, the notes files will be
somewhere within the libevdev/ directory, not the test/ directory. Working
around this in automake gets nasty quickly, so just add a script that knows
how to search for things.

No functional changes unless --enable-gcov is given at configure time - then
don't install the library.

The gcov reports are now in test/gcov-reports/

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Acked-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>