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.

HACKING

@@ -18,8 +18,11 @@ the code, you need the following tools:
     - lex/flex
     - yacc/bison
     - gperf
+
+    Optionally:
     - check
-    - optionally Doxygen
+    - lcov/genhtml
+    - Doxygen
 
 To check out the master branch, use: