When building FFMPEG in the MSYS environment under Windows, one
must not use forward slashes ('/') for command-line options.  It
appears that the MSYS shell interprets these as absolute paths and
then automatically rewrites them into equivalent Windows paths.  For
example, the '/nologo' switch below gets rewritten to something like
'C:/Program Files/Git/nologo', and this obviously breaks the build.
Thankfully, most M$ tools accept dashes ('-') as well.
Signed-off-by: Ziemowit Łąski <15880281+zlaski@users.noreply.github.com>
Signed-off-by: Martin Storsjö <martin@martin.st>