Add accidentally deleted BPFCFLAGS

configure.ac

@@ -556,6 +556,9 @@ if test "x${request_libbpf}" != "xno"; then
                     [have_libbpf=no])
   if test "x${have_libbpf}" = "xyes"; then
     AC_DEFINE([HAVE_LIBBPF], [1], [Define to 1 if you have `libbpf` library.])
+    if test "x${BPFCFLAGS}" = "x"; then
+      BPFCFLAGS="-Wall -O2"
+    fi
     # Add the include path for Debian
     EXTRABPFCFLAGS="-I/usr/include/$host_cpu-$host_os"
     AC_SUBST([EXTRABPFCFLAGS])