adapt Makefiles to work on my Windows setup
Use "sed" instead of "awk" for deleting to the end of the file

Avoid the additional processes for "awk" and also for "cat".
This also avoids having to add a workaround for the awk invocation failing on
Windows cmd.exe.
use 2x -e instead of separated by ; in 2 lines

working on Linux and mingw shell, but error on mingw cmd:
sed.exe: -e expression #2, char 1: Unknown command: `C'
use 2 commands separated by ; but in 1 line

working on mingw cmd and shell, I guess also in Linux
merge SDL2 flags for cmd.exe and MinGW shell
add .exe suffix so make knows that is is built

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, there
has to be some magic because ls shows AppNotes, although it doesn't exist.