libcurl: use c-ares if available

By default libcurl uses the C library's DNS resolver which is
synchronous, even if an application is using libcurl's non-blocking mode
of operation.

Configure libcurl to use c-ares if it is selected so that it can resolve
addresses asynchronously if required.

[Peter: explicitly disable c-ares support if not enabled]
Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/libcurl/libcurl.mk

@@ -46,6 +46,13 @@ LIBCURL_CONF_OPTS += --without-ssl --without-gnutls \
 	--without-polarssl --without-nss
 endif
 
+ifeq ($(BR2_PACKAGE_C_ARES),y)
+LIBCURL_DEPENDENCIES += c-ares
+LIBCURL_CONF_OPTS += --enable-ares
+else
+LIBCURL_CONF_OPTS += --disable-ares
+endif
+
 # Configure curl to support libssh2
 ifeq ($(BR2_PACKAGE_LIBSSH2),y)
 LIBCURL_DEPENDENCIES += libssh2