would show a message as like
"checking wether gcc accepts -fvomit-frame ... no"
and sets the shell-variable $vomitframe to either "-fvomit-frame"
or (in this case) just a simple "". In many cases you would then call
AC_SUBST(_fvomit_frame_,$vomitframe) to create a substitution that
could be fed as "CFLAGS = @_funsigned_char_@ @_fvomit_frame_@
in consequence this function is much more general than their
specific counterparts like ac_cxx_rtti.m4 that will test for