aclocal.m4: call cygpath on mingw32 only

The only reason I noticed is warning these lines on linux:

```
$ ./configure --target=sparc-unknown-linux-gnu
...
./configure: line 9708: cygpath: command not found
./configure: line 9708: ArCmd: command not found
```

POSIX shell syntax requires no spaces in assignments.
Fixed guarding condition while at it.
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

aclocal.m4

@@ -1149,8 +1149,8 @@ else
   fi
 fi])
 fp_prog_ar_args=$fp_cv_prog_ar_args
-if test "$HostOS" != "mingw32"; then
-    ArCmd = "$(cygpath -m $ArCmd)"
+if test "$HostOS" = "mingw32"; then
+    ArCmd=$(cygpath -m $ArCmd)
 fi
 AC_SUBST([ArCmd], ["$fp_prog_ar"])
 AC_SUBST([ArArgs], ["$fp_prog_ar_args"])