Hi Dennis,
Just to let you know you don't actually need to recompile the entirety of monetdb to create an extension.
You can place the shared library file containing your compiled udf into the directory containing the other monetdb modules (location dependent on platform ) along with an appropriate mal file and autoload entry.
I am on holiday at the minute but I did exactly this very recently for monetdb on ubuntu and can provide you with more details next week if you want.
Best regards,
Alastair