Heed and remove warning issued by autoconf 2.68.
Heed and remove warning issued by autoconf 2.68.

Starting in autoconf 2.68, "the macros AC_PREPROC_IFELSE,
AC_COMPILE_IFELSE, AC_LINK_IFELSE, and AC_RUN_IFELSE now warn if the
first argument failed to use AC_LANG_SOURCE or AC_LANG_PROGRAM to
generate the conftest file contents."

CastMi/iverilog,themperek/iverilog,CastMi/iverilog,themperek/iverilog,CastMi/iverilog,themperek/iverilog