Fix normalisation of path to find

The old code didn't work on Mac OS X, as "which" returns successfully
when it can't find the program.

aclocal.m4

@@ -611,11 +611,15 @@ echo foo > conftest.txt
 $fp_prog_find conftest.txt -print > conftest.out 2>&1
 if grep '^conftest.txt$' conftest.out > /dev/null 2>&1 ; then
   # OK, looks like a real "find".
-  if which cygpath 1> /dev/null 2> /dev/null
-  then
-    fp_prog_find=`cygpath --mixed "$fp_prog_find"`
-    AC_MSG_NOTICE([Converted to "$fp_prog_find"])
-  fi
+  case $HostPlatform in
+    *mingw32)
+      if test x${OSTYPE} != xmsys
+      then
+ 	    fp_prog_find="`cygpath --mixed ${fp_prog_find}`"
+        AC_MSG_NOTICE([normalized find command to $fp_prog_find])
+      fi ;;
+    *) ;;
+  esac
   FindCmd="$fp_prog_find"
 else
   # Found a poor WinDoze version of "find", ignore it.