Add --enable-coverage configure option

This configure flag enables lcov [1] coverage generation and is intended
to be used with unit tests (--enable-unit-tests is implied).

A html coverage report can be generated by issuing the following command
in the toplevel build directory:

make coverage

[1] - http://ltp.sourceforge.net/coverage/lcov.php

Based on a patch by Adrian-Ken Rueegsegger.

.gitignore

@@ -34,3 +34,7 @@ apidoc/
 *.tar.bz2
 *.tar.gz
 .DS_Store
+coverage/
+*.gcno
+*.gcda
+*.gcov