Print a warning when disabling http support for dependencies.

This should make mingw32-configure --enable-http less confusing.

configure.ac

@@ -40,7 +40,12 @@ AC_ARG_ENABLE([http],
   AS_HELP_STRING([--disable-http], [Disable HTTP support]),,
   enable_http=yes)
 
-AC_CHECK_HEADER([sys/socket.h],,[enable_http=no])
+AS_IF([test "x$enable_http" != "xno"],
+  AC_CHECK_HEADER([sys/socket.h],,
+    AC_MSG_WARN([HTTP support requires a posix socket library.])
+    enable_http=no
+  )
+)
 
 AS_IF([test "x$enable_http" != "xno"], [
    openssl="openssl"