-dnl We don't use the standard autoconf macros for this, but instead
-dnl ask the bootstrapping compiler what platform it is for
-
-dnl Guess host/target/build platform(s) if necessary.
-m4_divert_once([HELP_CANON],
-[[
-System types:
- --build=BUILD configure for building on BUILD [guessed]
- --host=HOST cross-compile to build programs to run on HOST [guessed]
- --target=TARGET configure for building compilers for TARGET [guessed]]])dnl
-
-if test "$build" = ""
+dnl If we aren't explicitly told what values to use with configure flags,
+dnl we ask the bootstrapping compiler what platform it is for
+
+if test "${WithGhc}" != ""
+then
+ bootstrap_host=`"${WithGhc}" +RTS --info | grep '^ ,("Host platform"' | sed -e 's/.*, "//' -e 's/")//' | tr -d '\r'`
+ bootstrap_target=`"${WithGhc}" +RTS --info | grep '^ ,("Target platform"' | sed -e 's/.*, "//' -e 's/")//' | tr -d '\r'`
+ if test "$bootstrap_host" != "$bootstrap_target"
+ then
+ echo "Bootstrapping GHC is a cross compiler. This probably isn't going to work"
+ fi
+fi
+
+# We have to run these unconditionally, but we may discard their
+# results in the following code
+AC_CANONICAL_BUILD
+AC_CANONICAL_HOST
+AC_CANONICAL_TARGET
+
+# If no argument was given for a configuration variable, then discard
+# the guessed canonical system and use the configuration of the
+# bootstrapping ghc. If an argument was given, map it from gnu format
+# to ghc format.
+#
+# For why we do it this way, see: #3637, #1717, #2951
+
+if test "$build_alias" = ""