configure: use AC_COMPILE_IFELSE() and AC_LANG_SOURCE() for testing flags.

[tcpdump] / aclocal.m4

diff --git a/aclocal.m4 b/aclocal.m4
index f12438aea29c7e672fd02905e302578aa59d7e21..c34a16b91e7f345d5ba426cfaeeb8905a28cddf1 100644 (file)
--- a/aclocal.m4
+++ b/aclocal.m4
@@ -232,40 +232,22 @@ AC_DEFUN(AC_LBL_CHECK_COMPILER_OPT,
        save_ac_c_werror_flag="$ac_c_werror_flag"
        ac_c_werror_flag=yes
        #
-       # XXX - with autoconf 2.69, at least, the test program that this
-       # tries to compile is:
+       # We use AC_LANG_SOURCE() so that we can control the complete
+       # content of the program being compiled.  We do not, for example,
+       # want the default "int main()" that AC_LANG_PROGRAM() generates,
+       # as it will generate a warning with -Wold-style-definition, meaning
+       # that we would treat it as not working, as the test will fail if
+       # *any* error output, including a warning due to the flag we're
+       # testing, is generated; see
        #
-       # int
-       # main ()
-       # {
+       #    https://www.postgresql.org/message-id/2192993.1591682589%40sss.pgh.pa.us
+       #    https://www.postgresql.org/message-id/2192993.1591682589%40sss.pgh.pa.us
        #
-       #   ;
-       #   return 0;
-       # }
+       # This may, as per those two messages, be fixed in autoonf 2.70,
+       # but we only require 2.64 or newer for now.
        #
-       # Hopefully, neither the empty statement nor the old-style
-       # definition of main() will, with any command-line flag
-       # whatsoever with which we test, on any compiler we test,
-       # will produce any warnings whatsoever; if it does, the
-       # command-line flag with which we test will be treated as
-       # not being supported even if it is supported.
-       #
-       # Thanks, autoconf, for making it *so* difficult to generate
-       # an absolute minimum valid C-with-everything-prototyped
-       # program as a test program, such as
-       #
-       #       int main(void) { return 0; }.
-       #
-       # (with autoconf 2.69, at least, using AC_LANG_CONFTEST() with
-       # AC_LANG_SOURCE([<code>]) produces the same function boilerplate
-       # as AC_LANG_PROGRAM([],[<code>]), complete with the main()
-       # function wrapper, the extra semicolon, and the return 0;,
-       # raising the question of "why, then, do both AC_LANG_SOURCE()
-       # and AC_LANG_PROGRAM() exist?").
-       #
-       AC_TRY_COMPILE(
-           [],
-           [],
+       AC_COMPILE_IFELSE(
+           [AC_LANG_SOURCE([[int main(void) { return 0; }]])],
            [
                AC_MSG_RESULT([yes])
                CFLAGS="$save_CFLAGS"