php: fix libxml2 build failure

The libxml2 build test uses the PHP_TEST_BUILD macro which in turn uses
AC_TRY_RUN which is bad for cross compilations.
Force php_cv_libxml_build_works to yes when libxml2 is selected. Fixes:
http://autobuild.buildroot.net/results/8b45a1260ab6ae15dc59a5d6b5b98698ec3e7bbe/

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

package/php/php.mk

@@ -66,6 +66,7 @@ ifeq ($(BR2_PACKAGE_PHP_EXT_OPENSSL),y)
 endif
 
 ifeq ($(BR2_PACKAGE_PHP_EXT_LIBXML2),y)
+	PHP_CONF_ENV += php_cv_libxml_build_works=yes
 	PHP_CONF_OPT += --enable-libxml --with-libxml-dir=${STAGING_DIR}/usr
 	PHP_DEPENDENCIES += libxml2
 endif