put generic checks at the beginning
in-code documentation
m4-macro to extend AC_INCLUDES_DEFAULT
add windows.h to the default-headers
don't force SDL/glfw to "yes"
including <winsock2.h> before <windows.h>
Removed libvlc_new() arguments

according to the online (doxygen) documentation, these arguments
should not be used except for debugging, as they can change
rapidly between releases
clamp imagename to 80chars

including the terminating NULL-byte.
this is according to the SGI-image specs.

furthermore, don't use strnlen() as this is not
available on w32.
workflow without github
note on "good commits"
drop git from CodingStyle

it's really in Gem/wiki/Contributing
Merge branch 'master' of git://git.code.sf.net/p/pd-gem/gem