If you already have a running GNU system, and only want to recompile the kernel, for example to select a different set of included hardware drivers, you can easily do this. You need the GNU C compiler and MIG, the Mach interface generator, which both come in their own packages.
Building and installing the kernel is as easy as with any other GNU software package. The configure script is used to configure the source and set the compile time options. The compilation is done by running:
To install the kernel and its header files, just enter the command:
This will install the kernel as EXEC_PREFIX/boot/gnumach, the header files into PREFIX/include/, the list of message ids as PREFIX/share/msgids/gnumach.msgids and the documentation into PREFIX/share/info/.
Note that there is also a way to only install the header and documentation files without having to actually build the whole package: run make install-data after having ran configure to do so. (This is needed for bootstrapping a cross compiler and similar procedures.)