Fix for minor Makefile issues

[ceeac]

This fixes several minor Makefile issues:

• Building and cleaning nettool, and cleaning makeobj from the main Makefile were not possible
• Clang does not support -fno-schedule-insns, so it makes no sense supplying it when building with clang
• COLOUR_DEPTH = 0 is equivalent to BACKEND = posix, so I enabled setting COLOUR_DEPTH automatically instead of manually in config.default
• When you edit the Makefile or config.default, make reports all targets as up-to-date, which is incorrect; instead, a full rebuild is necessary since compiler flags might have changed.

I also enabled building makeobj and nettool by default to catch possible compile errors early, but this can be easily disabled by modifying the all target.

[prissi]

Usually I clean after building simutrans before building makeobj, since the flags are different for the debug module simdebug.cc (at least that was the case for a long time). I am not sure building all in one will generate a working binary.

So the default build is to build simutrans only. Incorporated in r 9056.

NB: Since I updated Mingw, the make file and compiler runs at snails pace. I need nearly an hour to compile simutrans now.

EDIT: The svn call was done before each compilation thanks to quotes. Without it runs as fast as before ...