package/safeclib: fix build with gcc 4.8

Add -std=c99 to fix the following build failure with gcc 4.8 raised
since bump to version 3.7.1 in commit
cc27267ae41bf44276c63b1e86042c4fae4adf50:

In file included from abort_handler_s.c:35:0:
safeclib_private.h:167:18: error: anonymous variadic macros were introduced in C99 [-Werror=variadic-macros]
 #define slprintf(...) fprintf(stderr, __VA_ARGS__)
                  ^

Fixes:
 - http://autobuild.buildroot.org/results/5c3468585942879b47331e05058d25d324c8cc23

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

package/safeclib/safeclib.mk

@@ -11,6 +11,7 @@ SAFECLIB_SOURCE = safeclib-$(SAFECLIB_VERSION).tar.xz
 SAFECLIB_LICENSE = MIT
 SAFECLIB_LICENSE_FILES = COPYING
 SAFECLIB_INSTALL_STAGING = YES
+SAFECLIB_CONF_ENV = CFLAGS="$(TARGET_CFLAGS) -std=c99"
 # -fstack-protector-strong is used by default. Disable that so the
 # BR2_SSP_* options in the toolchain wrapper are used instead
 SAFECLIB_CONF_OPTS = --disable-hardening