The project files are available via svn only for now.
To get the source use:
svn co http://libgarmin.svn.sourceforge.net/svnroot/libgarmin/libgarmin/dev libgarmin
build with:
./autosh.sh
./configure
make
make install

How to build Navit driver:
Driver is merged into navit CVS so you do not have to patch it, just install the library and rebuild navit.
Then you can add maps to navit.xml

        <mapset enabled="yes">
                <map type="garmin" enabled="yes" data="/path/to/gmapsupp.img"/>
        </mapset>

Where /path/to/gmapsupp.img is a dskimg file or a .tdb file

Obsolete instructions:

Get these Navit patches:
Add Garmin types
Navit's configure script
Navit Makefiles
Apply with:
cd navitsrcdir
cat patchname | patch -p1
If you got rejects you have to fix them.
Next go to
cd navitsrcdir/src/data
And checkout navit's driver
svn co http://libgarmin.svn.sourceforge.net/svnroot/libgarmin/navitdrv/dev garmin
Next reconfigure and build navit.
Now add your first garmin map to navit.xml

        <mapset enabled="yes">
                <map type="garmin" enabled="yes" data="/path/to/gmapsupp.img"/>
        </mapset>

Next install garmin types definitions:
cp navitsrcdir/src/data/garmin/garmintypes.txt /path/to/

For more visit project Wiki:libgarmin wiki
or email libgarmin at gmail dot com
SourceForge Project Page