I have been able to find the manual for MPlib on CTAN, but where can I find the C sources for the library itself? Sorry for asking such a basic question.
The project is hosted on FusionForge. You can obtain the source code with Subversion using:
svn checkout https://foundry.supelec.fr/svn/metapost trunk
Note the https! The old URL from my comment above is not valid any longer since FusionForge restructured their site.