Need --enable-code-coverage option in "configure"

[Michel Van Eeckhout]

We need to add a "--enable-code-coverage" option in "configure" that adds compiling CFLAGS options "-fprofile-arcs" and "-ftest-coverage" for gcc.
When compiling with those options, the resulting binary files generate code coverage statistics for each execution. This is great to measure the extent of our unit tests. When running "gcov" on those statistics, you can produce an annotated source file that tells you which lines were executed and which were not. You can then add tests to cover the lines you missed.