Source out common Makefile parts into an included file
Ignore non-existent files when removing in the clean/distclean targets
Use "sed" instead of "awk" for deleting to the end of the file

Avoid the additional processes for "awk" and also for "cat".

The two sed commands have been integrated in one -e expression to avoid
the problem with the leading slash / being translated to C: when invoked
in Windows cmd.exe, which was the original reason for this change.
Remove trailing slash from directory passed to mkdir

This caused the following error with mkdir from UnxUtils on Windows,
even if -p was given:

    mkdir.exe: cannot create directory `build\': File exists
Add SDL2 compiler/linker flags for Windows builds

A simple mechanism for system detection has been added.
Handle the "weak" attribute in MLA file preparation

Some workarounds necessary for Windows, where this attribute is
unsupported.
SDL: use software renderer

On a system running WinXP 32 bit the accelerated renderer did not work
properly, so always use the software renderer, which should always work
I guess.

Performance is not an issue here, reliability is more important.
Add .exe file extension for Windows build

For the native Windows build it was automatically added, but the
Makefile assumed it without suffix and thus there was never "Nothing do
be done". In MinGW Shell however it worked, apparently there has to be
some magic because ls shows AppNotes, although it doesn't exist.