package/hiawatha: fix build with gcc 4.8

Fix the following build failure raised since commit
4e36e8f2081a66f477dd038793082045876d65ee:

/tmp/instance-1/output-1/build/hiawatha-11.1/mbedtls/library/ecjpake.c: In function 'self_test_rng':
/tmp/instance-1/output-1/build/hiawatha-11.1/mbedtls/library/ecjpake.c:973:5: error: 'for' loop initial declarations are only allowed in C99 mode
     for( size_t i = 0; i < len; i++ )
     ^

Fixes:
 - http://autobuild.buildroot.org/results/1befc1552af91f34c600229138910c2078c2fdca

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

package/hiawatha/hiawatha.mk

@@ -15,6 +15,7 @@ HIAWATHA_CPE_ID_VENDOR = hiawatha-webserver
 HIAWATHA_CONF_OPTS = \
 	-DINSTALL_MBEDTLS_HEADERS=OFF \
 	-DUSE_SYSTEM_MBEDTLS=OFF \
+	-DCMAKE_C_FLAGS="$(TARGET_CFLAGS) -std=c99" \
 	-DENABLE_TOOLKIT=OFF \
 	-DCONFIG_DIR=/etc/hiawatha \
 	-DLOG_DIR=/var/log \