Issue #27229: Merge cross-compiling fix from 3.5

Makefile.pre.in

@@ -793,13 +793,16 @@ $(GRAMMAR_H): $(GRAMMAR_INPUT) $(PGEN)
 	if test "$(cross_compiling)" != "yes"; then \
 		$(PGEN) $(GRAMMAR_INPUT) $(GRAMMAR_H) $(GRAMMAR_C); \
 	else \
-		cp $(srcdir)/Include/graminit.h $(GRAMMAR_H); \
+		# Avoid copying the file onto itself for an in-tree build \
+		cp $(srcdir)/Include/graminit.h $(GRAMMAR_H).tmp; \
+		mv $(GRAMMAR_H).tmp $(GRAMMAR_H); \
 	fi
 $(GRAMMAR_C): $(GRAMMAR_H)
 	if test "$(cross_compiling)" != "yes"; then \
 		touch $(GRAMMAR_C); \
 	else \
-		cp $(srcdir)/Python/graminit.c $(GRAMMAR_C); \
+		cp $(srcdir)/Python/graminit.c $(GRAMMAR_C).tmp; \
+		mv $(GRAMMAR_C).tmp $(GRAMMAR_C); \
 	fi
 
 $(PGEN): $(PGENOBJS)

Misc/NEWS

@@ -137,6 +137,9 @@ Windows
 Build
 -----
 
+- Issue #27229: Fix the cross-compiling pgen rule for in-tree builds.  Patch
+  by Xavier de Gaye.
+
 - Issue #26930: Update OS X 10.5+ 32-bit-only installer to build
   and link with OpenSSL 1.0.2h.