This is due to drive letters not being available in POSIX OSes (e.g. Linux), hence you must patch the sources to realize that __DJGPP__ sometimes is similar to _WIN32 and rebuild. Though I don't recommend that, honestly.

Or just use the workaround (mentioned in other bug report) of "7za a whatever /dev/c/mydir/*.c" instead. In other words, you must use DJGPP-style drive syntax instead. Klunky but works okay.

Or use Win32's 7ZA.EXE with Japheth's HX. I haven't tried lately, but it should still work.

http://sourceforge.net/projects/sevenzip/files/7-Zip/9.20/7za920.zip/download
http://www.japheth.de/HX.html