Actually, the trouble is that OpenOCD is not consistently using the m4_version_prereq solution. You're getting the message from the unprotected call to AC_PROG_CC_C99 in openocd-code/src/jtag/drivers/libjaylink/configure.ac.
I hadn't realised, many thanks for your help. That file actually belongs to a Git Submodule, so that is a different project. I'll let the maintainer know. Regards, rdiez
