mingw fixes, enable build dll w/ mingw (SF#2000973: https://sourceforge.net/tracker2/?func=detail&aid=2000973&group_id=13478&atid=313478)

configure.in

@@ -25,6 +25,9 @@ AM_INIT_AUTOMAKE(flac, 1.2.1)
 # explicitly requested.  Eases autobuilding -mdz
 AM_MAINTAINER_MODE
 
+# Enable the generation of shared libraries under Win32
+AC_LIBTOOL_WIN32_DLL
+
 # We need two libtools, one that builds both shared and static, and
 # one that builds only static.  This is because the resulting libtool
 # does not allow us to choose which to build at runtime.
@@ -93,11 +96,19 @@ case "$host" in
 esac
 AC_SUBST(OBJ_FORMAT)
 
-# only needed because of ntohl() usage, can get rid of after that's gone:
 case "$host" in
-	*-*-cygwin|*mingw*) MINGW_WINSOCK_LIBS=-lwsock32 ;;
-	*) MINGW_WINSOCK_LIBS= ;;
+	*-*-cygwin|*mingw*)
+		# define this variable for enabling strict exports with libtool; for now, it's only supported by Win32
+		LT_NO_UNDEFINED="-no-undefined"
+		# -lwsock32 only needed because of ntohl() usage, can get rid of after that's gone:
+		MINGW_WINSOCK_LIBS=-lwsock32
+		;;
+	*)
+		LT_NO_UNDEFINED=
+		MINGW_WINSOCK_LIBS=
+		;;
 esac
+AC_SUBST(LT_NO_UNDEFINED)
 AC_SUBST(MINGW_WINSOCK_LIBS)
 
 case "$host" in