Bug 557: config.h misses INSTALL_SUBDIR_* variables on first cmake run

Bug 557: config.h misses INSTALL_SUBDIR_* variables on first cmake run

The first time cmake runs in a new supertux build tree, config.h is generated
before the INSTALL_SUBDIR_{BIN,SHARE} variables are set and thus contains empty
values for these variables. If the executable is built right away and run, it
crashes because it cannot find the data files.

Resolves #557. Thanks to Matt McCutchen for this patch.

git-svn-id: http://supertux.lethargik.org/svn/supertux/trunk/supertux@6293 837edb03-e0f3-0310-88ca-d4d4e8b29345