Fix MinGW shared, PIC and rdynamic flags.

A native MinGW compile was not setting these to the correct
value. This patch makes them match what Cygwin uses. Adding
the -Wl,--enable-auto-image-base should make vvp load
slightly faster since it loads multiple DLL files.

aclocal.m4

@@ -105,6 +105,10 @@ case "${host}" in
         shared="-shared -Wl,--enable-auto-image-base"
         ;;
 
+     *-*-mingw*)
+        shared="-shared -Wl,--enable-auto-image-base"
+        ;;
+
      *-*-hpux*)
         shared="-b"
         ;;
@@ -134,6 +138,10 @@ case "${host}" in
         PICFLAG=
         ;;
 
+     *-*-mingw*)
+        PICFLAG=
+        ;;
+
      *-*-hpux*)
         PICFLAG=+z
         ;;
@@ -169,6 +177,10 @@ case "${host}" in
         rdynamic=""
         ;;
 
+    *-*-mingw*)
+        rdynamic=""
+        ;;
+
     *-*-hpux*)
         rdynamic="-E"
         ;;