compat: Use '-' instead of '/' for rc.exe options

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>

compat/windows/mswindres

 #!/bin/sh
 
 if [ "$1" = "--version" ]; then
-    rc.exe /?
+    rc.exe -?
     exit $?
 fi
 
@@ -10,12 +10,12 @@ if [ $# -lt 2 ]; then
     exit 0
 fi
 
-EXTRA_OPTS="/nologo"
+EXTRA_OPTS="-nologo"
 
 while [ $# -gt 2 ]; do
     case $1 in
-    -D*) EXTRA_OPTS="$EXTRA_OPTS /d$(echo $1 | sed -e "s/^..//" -e "s/ /\\\\ /g")" ;;
-    -I*) EXTRA_OPTS="$EXTRA_OPTS /i$(echo $1 | sed -e "s/^..//" -e "s/ /\\\\ /g")" ;;
+    -D*) EXTRA_OPTS="$EXTRA_OPTS -d$(echo $1 | sed -e "s/^..//" -e "s/ /\\\\ /g")" ;;
+    -I*) EXTRA_OPTS="$EXTRA_OPTS -i$(echo $1 | sed -e "s/^..//" -e "s/ /\\\\ /g")" ;;
     -o)  OPT_OUT="$2"; shift ;;
     esac
     shift
@@ -29,4 +29,4 @@ else
 fi
 
 eval set -- $EXTRA_OPTS
-rc.exe "$@" /fo "$OUT" "$IN"
+rc.exe "$@" -fo "$OUT" "$IN"