Bug 2426

You can now build libdrm as a shared object, by snagging it from cvs and doing:

$ cd libdrm && make && sudo make install

This is super cool because we can eliminate libdrm being duplicated in three
places (drm, Mesa, and Xorg), share libdrm between server and client, shrink the
driver footprint a bit, etc.

This patch switches the DRI bits to link against libdrm.so.  This is a flag day;
after applying this patch DRI _must_ have libdrm already built in order to
build.  (Non-DRI builds of Mesa are unaffected.)  This also means we need to add
libdrm to the snapshots, update Building, etc.  With libdrm kicked out and the
includes cleaned up a bit this drops over 4000 lines from Mesa.

In my testing this is completely binary-compatible in both directions
(shared-libdrm driver with static-libdrm libGL, and vice versa).  Untested on
Solo but should work the same as with X.  Please test.