cmake: fix build on MSYS2

Some minor changes to allow building easily on MSYS2:

- don't check for snorenotify on MSYS2 because it segfaults on startup

- don't use -fstack-protector-strong (libssp) on MinGW because it
currently causes segfaults during static initialization

https://sourceforge.net/p/mingw-w64/bugs/755/

- use the `cat` command to read files when using the 'MSYS Makefiles'
generator, and not the cmd.exe `type` command

- update doc for the quassel_add_module function wrt. 7d0879ab

Tested to build correctly on MSYS2 with both the 'MSYS Makefiles'
generator and the Ninja generator using a native ninja.

Signed-off-by: Rafael Kitover <rkitover@gmail.com>