dnl AC_ECHO_MKFILE(MSG, FILE)
dnl create the directory path to file, if any
dnl and output the message as the first line to that file
dnl thereby truncating any other content if there was any.
dnl
dnl WARNING: update this macro for 2.49, see the helper functions in use
dnl i.e. AC_AS_DIRNAME and AC_AS_MKDIR_P

