gh-112305: Fix check-clean-src to detect frozen_modules .h files. (#113344)

A typo left this check broken so many of us who do out-of-tree builds
were seeing strange failures due to bad `Python/frozen_modules/*.h`
files being picked up from the source tree and used at build time from
different Python versions leading to errors like:

`Fatal Python error: _PyImport_InitCore: failed to initialize importlib`

Or similar once our build got to an "invoke the interpreter"
bootstrapping step due to incorrect bytecode being embedded.

Makefile.pre.in

@@ -657,11 +657,13 @@ check-clean-src:
 	@if test -n "$(VPATH)" -a \( \
 	    -f "$(srcdir)/$(BUILDPYTHON)" \
 	    -o -f "$(srcdir)/Programs/python.o" \
-	    -o -f "$(srcdir)\Python/frozen_modules/importlib._bootstrap.h" \
+	    -o -f "$(srcdir)/Python/frozen_modules/importlib._bootstrap.h" \
 	\); then \
 		echo "Error: The source directory ($(srcdir)) is not clean" ; \
 		echo "Building Python out of the source tree (in $(abs_builddir)) requires a clean source tree ($(abs_srcdir))" ; \
-		echo "Try to run: make -C \"$(srcdir)\" clean" ; \
+		echo "Build artifacts such as .o files, executables, and Python/frozen_modules/*.h must not exist within $(srcdir)." ; \
+		echo "Try to run:" ; \
+		echo "  (cd \"$(srcdir)\" && make clean || git clean -fdx -e Doc/venv)" ; \
 		exit 1; \
 	fi
 

Misc/NEWS.d/next/Build/2023-12-21-05-35-06.gh-issue-112305.VfqQPx.rst

@@ -0,0 +1,3 @@
+Fixed the ``check-clean-src`` step performed on out of tree builds to detect
+errant ``$(srcdir)/Python/frozen_modules/*.h`` files and recommend
+appropriate source tree cleanup steps to get a working build again.