configure: Don't add -fPIC on windows targets

This avoids warnings about this option not having any effect on
this platform.

We still want to enable the pic configure item for these platforms
(if detected via the compiler builtin define __PIC__) to get proper
inline assembly workarounds.
Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -3350,7 +3350,13 @@ enable_weak_pic() {
     disabled pic && return
     enable pic
     add_cppflags -DPIC
-    add_cflags   -fPIC
+    case "$target_os" in
+    mingw*|cygwin*)
+        ;;
+    *)
+        add_cflags -fPIC
+        ;;
+    esac
     add_asflags  -fPIC
 }