package/liburcu: fix build without wchar

Fix the following build failure raised since bump to version 0.13.0 in
commit 9cedbcf494e647371f92368014d31091848772bb and
https://github.com/urcu/userspace-rcu/commit/8b2f0590d94f6bc3f1d31bd3e0c9ea744b5b55cb:

configure: error: The compiler does not support C99

Fixes:
 - http://autobuild.buildroot.org/results/435d127728b014d565dd75861f87e18716f9fe5e

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/liburcu/liburcu.mk

@@ -12,4 +12,8 @@ LIBURCU_LICENSE_FILES = lgpl-2.1.txt lgpl-relicensing.txt gpl-2.0.txt LICENSE
 
 LIBURCU_INSTALL_STAGING = YES
 
+# ac_cv_prog_cc_c99 is required for BR2_USE_WCHAR=n because the C99 test
+# provided by autoconf relies on wchar_t.
+LIBURCU_CONF_ENV = ac_cv_prog_cc_c99=-std=gnu99
+
 $(eval $(autotools-package))