Move MinGW mixed path conversion code to the pathconvert tool

In preparation for adding MinGW support to patch #438.

Tools/convertpath

@@ -1,25 +1,32 @@
 #!/bin/sh
 
-# Unix to Windows relative path conversion in a script.
+# Unix to Windows path conversion in a script.
 # Useful for avoiding backslash quoting difficulties in Makefiles.
-# Acts as a much dumbed down 'cygpath -w' tool.
+# Acts as a much dumbed down cygpath tool mainly for use on MinGW.
 
 usage()
 {
   cat <<EOF
-Usage: $0 [-w|-u|-h] [path]...
+Usage: $0 [-m|-u|-w|-h] [path]...
 
-Convert Windows and Unix paths (intended for simple relative paths only)
+Convert Windows and Unix paths
 
-  -w  Convert backslashes to forward slashes in the paths
-  -u  Convert forward slashes to backslashes in the paths
+  -m  Convert Unix path to mixed path (full paths, MinGW MSYS only)
+  -u  Convert forward slashes to backslashes in the paths (relative paths only)
+  -w  Convert backslashes to forward slashes in the paths (relative paths only)
 EOF
 }
 
 option="$1"
 case "$option" in
-  -w) shift; echo $@ | sed -e 's,/,\\,g' ;;
+  -m) shift
+      case $MACHTYPE in
+        # This echo converts unix to mixed paths. Then zap unexpected trailing space on old versions of MinGW.
+        *-msys) echo `cmd //c echo "$@" | sed -e "s/[ ]*$//"`;;
+        *) echo "The -m option is only supported on MinGW MSYS" 1>&2; exit 1 ;;
+      esac ;;
   -u) shift; echo $@ | sed -e 's,\\,/,g' ;;
+  -w) shift; echo $@ | sed -e 's,/,\\,g' ;;
   -h) shift; usage; ;;
   *) usage; exit 1 ;;
 esac