Add configure option --enable-unit-tests

This configure option enables check-based unit testing. Check is a unit
test framework for C [1].

The unit tests can be executed by issuing the following command in the
toplevel build directory:

make check

[1] - http://check.sourceforge.net/

HACKING

@@ -18,6 +18,7 @@ the code, you need the following tools:
     - lex/flex
     - yacc/bison
     - gperf
+    - check
     - optionally Doxygen
 
 To check out the master branch, use: