Add LCOV support to generate coverage information

Add LCOV support to generate coverage information

How to use:

- Install LCOV (http://ltp.sourceforge.net/coverage/lcov.php)
- AFAICT, compiler must be GCC.
- ./configure --prefix=/some/path --enable-lcov=/var/www

  This will configure the sources so that generated coverage information
  (HTML pages) will end up in /var/www/coverage-report. Choose a directory
  you have write access to.

- make

  No installation necessary (although it probably doesn't hurt to have
  WM installed in ${prefix} so it can find resources and stuff, but the
  build tree must be kept intact anyway).

- Put ${srcroot}/src, ${srcroot}/WPrefs.app and ${srcroot}/util first in
  your path.
- Arrange for your .xinitrc or DM to start ${srcroot}/src/wmaker as your
  window manager.
- Use it as you normally would, possibly for an extended period of time.
- make coverage.

  This will create a tree of web pages under /var/www/coverage-report

- Further `make coverage's (after some more use) will re-generate the
  report (deleting the directory first).

- `make coverage-reset' will reset execution counts to zero (see lcov(1)).

Signed-off-by: Tamas TEVESZ <ice@extreme.hu>