socat specifies -Werror when running some of its configure checks, and uses code that generates warnings (which are turned into errors).  This is extremely brittle because the code snippets used for checking tend to generate other warnings as well, rendering the checks ineffective.

An example is the AC_BASIC_TYPE macro below.  Some compilers warn about the useless comparison &u==&v, it seems.  GCC might do so eventually, too, so this is really brittle and should be fixed.

dnl find what physical type (basic C type) is equivalent to the given type.
dnl arg1: include file(s)
dnl arg2: type name
dnl arg3: output variable
dnl arg4: cache variable (might be constructed automatically)
dnl   output values: 1..short, 2..unsigned short, 3..int, 4..u-int,
dnl                  5..long, 6..u-long; others not yet supported
define(AC_BASIC_TYPE,[
AC_CACHE_CHECK(for equivalent simple type of $2, $4,
[CFLAGS1="$CFLAGS"; CFLAGS="$ERRONWARN $(echo "$CFLAGS1" | sed -e 's@-Wall@@g')"
 AC_TRY_COMPILE([$1],[$2 u; short v; &u==&v;],
 [$4="1 /* short */"],
 [AC_TRY_COMPILE([$1],[$2 u; unsigned short v; &u==&v;],
  [$4="2 /* unsigned short */"],
  [AC_TRY_COMPILE([$1],[$2 u; int v; &u==&v;],
   [$4="3 /* int */"],
   [AC_TRY_COMPILE([$1],[$2 u; unsigned int v; &u==&v;],
    [$4="4 /* unsigned int */"],
    [AC_TRY_COMPILE([$1],[$2 u; long v; &u==&v;],
     [$4="5 /* long */"],
     [AC_TRY_COMPILE([$1],[$2 u; unsigned long v; &u==&v;],
      [$4="6 /* unsigned long */"],
      [AC_TRY_COMPILE([$1],[$2 u; long long v; &u==&v;],
       [$4="7 /* long long */"],
       [AC_TRY_COMPILE([$1],[$2 u; unsigned long long v; &u==&v;],
        [$4="8 /* unsigned long long */"],
        [$4="0 /* unknown, taking default */"
]) ]) ]) ]) ]) ]) ]) ])
 CFLAGS="$CFLAGS1" ])
AC_DEFINE_UNQUOTED($3, ${$4})
])