Use an absolute path for SConsignFile so all Sconscript invocations share the same database

SConscript("some SCsub path") calls create a new context where a relative path
to the .sconsign file is interpreted as being next to the currently executing
SCsub, and not the one at the root. This breaks incremental build detection
because scons can't find the build information of dependent files outside of the
SCsub directory and just rebuilds everything every time.

SConstruct

@@ -152,7 +152,7 @@ env_base["x86_libtheora_opt_gcc"] = False
 env_base["x86_libtheora_opt_vc"] = False
 
 # avoid issues when building with different versions of python out of the same directory
-env_base.SConsignFile(".sconsign{0}.dblite".format(pickle.HIGHEST_PROTOCOL))
+env_base.SConsignFile(File("#.sconsign{0}.dblite".format(pickle.HIGHEST_PROTOCOL)).abspath)
 
 # Build options