dedsvr: Add libpng to link target in GNUMakefile

Also fixed some codes to build with gcc.
Specifically, png.cpp is created to separate PNG adaptation codes in
material.cpp which depends on JPEG and Windows bitmaps.  I don't need
them yet in the dedicated server.
dedsvr: Temporarily disable gltestdll.so
dedsvr: Make Warpable.cpp build with gcc
dedsvr: Add RStation.cpp to build target and make it compile with gcc
Expose Warpable::warpSpeed to Squirrel scripts
Merge branch 'clib' into HEAD
Fix a comment in StarEnum constructor
Prevent StarCaches from growing limitlessly

The galaxy contains finite but very large number of sectors. A sector
is currently 15 ly^3, which means there are about 4.0e5 sectors in the
galaxy.  Although the player won't have time to look at all of them,
it's no use remembering every cached sector.  We will forget about
old cached sectors after 1000 sectors are remembered, excluding sectors
containing materialized solar systems.  Forgetting materialized sectors
cause the solar system to re-materialize when the player warps to there.

The logic to determine the oldest sector could be improved to use last
accessed order instead of creation order.