configure: deprecate builds with small curl_off_t

If curl_off_t turns out to be smaller than 8 bytes,
--with-n64-deprecated needs to be used to allow the build to
continue. This is to highlight the fact that support for such builds is
going away next year.

Also mentioned in DEPRECATED.md

Closes #9605

configure.ac

@@ -3388,6 +3388,18 @@ AC_CHECK_TYPE(long long,
 )
 
 
+if test ${ac_cv_sizeof_curl_off_t} -lt 8; then
+  AC_ARG_WITH(n64-deprecated,dnl
+AS_HELP_STRING([--with-n64-deprecated],[confirm you realize support for systems without 64 bit data types is going away]),
+  if test X"$withval" != Xno; then
+    OPT_N64_AWARE=$withval
+  fi
+)
+  if test -z "$OPT_N64_AWARE"; then
+    AC_MSG_ERROR([support for systems without 64 bit curl_off_t is deprecated, use --with-n64-deprecated until then])
+  fi
+fi
+
 # check for ssize_t
 AC_CHECK_TYPE(ssize_t, ,
    AC_DEFINE(ssize_t, int, [the signed version of size_t]))

docs/DEPRECATE.md

@@ -23,6 +23,10 @@ compilers since the 1990s.
 
 Starting in 8.0.0 (March 2023), the plan is to drop support.
 
+Starting in 7.86.0, building curl with configure requires the additional flag
+`--with-n64-deprecated` if the `curl_off_t` type on your system is smaller
+than 8 bytes, in an attempt to highlight these plans to affected users.
+
 ## NSS
 
 We remove support for building curl with the NSS TLS library in August 2023.