Set C compiler options -Wextra -std=gnu99

scripts/build.sh

@@ -1,7 +1,8 @@
 #!/bin/sh
 set -eu
 CC="${CC:-clang}"
-CFLAGS="-O2 -falign-functions -Wall -Wno-strict-aliasing"
+CFLAGS="-Wall -Wextra -Wno-strict-aliasing -std=gnu99"
+CFLAGS="$CFLAGS -O2 -falign-functions"
 CFLAGS="$CFLAGS -I ../c -D NDEBUG -D USE_COMPUTED_GOTO"
 LFLAGS="-lm"
 builddir="build-$(uname | tr A-Z- a-z_)-$(uname -m | tr A-Z- a-z_)"