package/netdata: fix uclibc build without wchar

Fix the following uclibc build failure without wchar raised since bump
to version 1.33.1 in commit 73dc2eef2dc40b78e732872b26eee0bcea1087d1:

configure: error: Netdata requires a compiler that supports C99 to build

Fixes:
 - http://autobuild.buildroot.org/results/bca4d370ed0553d5f99f1277d0a1e3b49f62b95f

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

package/netdata/netdata.mk

@@ -18,6 +18,10 @@ NETDATA_CONF_OPTS = \
 	--disable-unit-tests
 NETDATA_DEPENDENCIES = libuv util-linux zlib
 
+# ac_cv_prog_cc_c99 is required for BR2_USE_WCHAR=n because the C99 test
+# provided by autoconf relies on wchar_t.
+NETDATA_CONF_ENV = ac_cv_prog_cc_c99=-std=gnu99
+
 ifeq ($(BR2_GCC_ENABLE_LTO),y)
 NETDATA_CONF_OPTS += --enable-lto
 else