build: Add required define for Windows

This will also add it to the .pc file as WEBRTC_WIN leaks into the
public interface and undefined __STRICT_ANSI__ so M_PI is available.

https://bugs.freedesktop.org/show_bug.cgi?id=96754

configure.ac

@@ -65,8 +65,11 @@ AS_CASE(["${host}"],
              [OS_FLAGS="-DWEBRTC_MAC -DWEBRTC_IOS"],
              [OS_FLAGS="-DWEBRTC_MAC"])
          PLATFORM_CFLAGS="-DWEBRTC_POSIX"
+        ],
+    [*-mingw32*],
+        [
+         PLATFORM_CFLAGS="-DWEBRTC_WIN -U__STRICT_ANSI__"
         ]
-    # FIXME: Add Windows support
 )
 AC_SUBST(PLATFORM_CFLAGS)