bpo-45886: Fix OOT build when srcdir has frozen module headers (GH-29793)

The presence of frozen module headers in srcdir interfers with OOT
build. Make considers headers in srcdir up to date, but later builds do
not use VPATH to locate files. make clean now removes the headers, too.

Also remove stale ``_bootstrap_python`` from .gitignore.

Author: tiran

.gitignore

@@ -60,7 +60,6 @@ Lib/distutils/command/*.pdb
 Lib/lib2to3/*.pickle
 Lib/test/data/*
 !Lib/test/data/README
-/_bootstrap_python
 /Makefile
 /Makefile.pre
 Mac/Makefile

Makefile.pre.in

@@ -587,7 +587,10 @@ build_all:	check-clean-src $(BUILDPYTHON) oldsharedmods sharedmods gdbhooks \
 
 # Check that the source is clean when building out of source.
 check-clean-src:
-	@if test -n "$(VPATH)" -a -f "$(srcdir)/Programs/python.o"; then \
+	@if test -n "$(VPATH)" -a \( \
+	    -f "$(srcdir)/Programs/python.o" \
+	    -o -f "$(srcdir)\Python/frozen_modules/importlib._bootstrap.h" \
+	\); 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" ; \
@@ -2293,6 +2296,8 @@ clean-retain-profile: pycremoval
 	-rm -f Lib/lib2to3/*Grammar*.pickle
 	-rm -f Programs/_testembed Programs/_freeze_module
 	-rm -f Python/deepfreeze/*.[co]
+	-rm -f Python/frozen_modules/*.h
+	-rm -f Python/frozen_modules/MANIFEST
 	-find build -type f -a ! -name '*.gc??' -exec rm -f {} ';'
 	-rm -f Include/pydtrace_probes.h
 	-rm -f profile-gen-stamp
@@ -2330,8 +2335,6 @@ distclean: clobber
 		Modules/Setup.stdlib Modules/ld_so_aix Modules/python.exp Misc/python.pc \
 		Misc/python-embed.pc Misc/python-config.sh
 	-rm -f python*-gdb.py
-	-rm -f Python/frozen_modules/*.h
-	-rm -f Python/frozen_modules/MANIFEST
 	# Issue #28258: set LC_ALL to avoid issues with Estonian locale.
 	# Expansion is performed here by shell (spawned by make) itself before
 	# arguments are passed to find. So LC_ALL=C must be set as a separate