Revert "configure.ac: add header check for stdarg.h"

This reverts commit b62b230578a3a8a748c22db91312dd0d6248d9e6.
protobuf-c/protobuf-c.c: replace assertions with error returns
protobuf-c/protobuf-c.c: do not print UNPACK_ERRORS by default (fixes #26)