Cross compiling needs host and build settings. configure no longer

creates a broken PYTHON_FOR_BUILD variable when --build is missing.

Misc/NEWS

@@ -467,6 +467,9 @@ Tests
 Build
 -----
 
+- Cross compiling needs host and build settings. configure no longer
+  creates a broken PYTHON_FOR_BUILD variable when --build is missing.
+
 - Fix cross compiling issue in setup.py, ensure that lib_dirs and inc_dirs are
   defined in cross compiling mode, too.
 

configure

@@ -2943,6 +2943,8 @@ $as_echo_n "checking for python interpreter for cross build... " >&6; }
 $as_echo "$interp" >&6; }
 	PYTHON_FOR_BUILD="_PYTHON_PROJECT_BASE=$srcdir"' _PYTHON_HOST_PLATFORM=$(_PYTHON_HOST_PLATFORM) PYTHONPATH=$(srcdir)/Lib:$(srcdir)/Lib/plat-$(MACHDEP) '$interp
     fi
+elif test "$cross_compiling" = maybe; then
+    as_fn_error $? "Cross compiling required --host=HOST-TUPLE and --build=ARCH" "$LINENO" 5
 else
     PYTHON_FOR_BUILD='./$(BUILDPYTHON) -E'
 fi

configure.ac

@@ -67,6 +67,8 @@ if test "$cross_compiling" = yes; then
         AC_MSG_RESULT($interp)
 	PYTHON_FOR_BUILD="_PYTHON_PROJECT_BASE=$srcdir"' _PYTHON_HOST_PLATFORM=$(_PYTHON_HOST_PLATFORM) PYTHONPATH=$(srcdir)/Lib:$(srcdir)/Lib/plat-$(MACHDEP) '$interp
     fi
+elif test "$cross_compiling" = maybe; then
+    AC_MSG_ERROR([Cross compiling required --host=HOST-TUPLE and --build=ARCH])
 else
     PYTHON_FOR_BUILD='./$(BUILDPYTHON) -E'
 fi