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It is common for Fink/OpenDarwin or if you compile Avidemux as a simple user. Assuming the extras are in /sw, do
export CFLAGS="-I/sw/include" export CXXFLAGS=$CFLAGS export CPPFLAGS=$CFLAGS export LDFLAGS="-L/sw/lib"
You can add as much as you want.
If you get complaints about xvid_2pass2 (Xvid 2(nbsp)pass), it usually means you have both XviD 0.9.x and 1.0.x on your system. Remove all but one, redo a configure and continue. The newer version is recommended.