gh-84461: Fix circulare dependency on BUILDPYTHON (GH-93977)

Makefile.pre.in

@@ -290,8 +290,11 @@ HOSTRUNNER= @HOSTRUNNER@
 PYTHON_FOR_REGEN?=@PYTHON_FOR_REGEN@
 UPDATE_FILE=$(PYTHON_FOR_REGEN) $(srcdir)/Tools/scripts/update_file.py
 PYTHON_FOR_BUILD=@PYTHON_FOR_BUILD@
+# Single-platform builds depend on $(BUILDPYTHON). Cross builds use an
+# external "build Python" and have an empty PYTHON_FOR_BUILD_DEPS.
+PYTHON_FOR_BUILD_DEPS=@PYTHON_FOR_BUILD_DEPS@
 
-# Normal builds use Programs/_freeze_module.c for bootstrapping and
+# Single-platform builds use Programs/_freeze_module.c for bootstrapping and
 # ./_bootstrap_python Programs/_freeze_module.py for remaining modules
 # Cross builds use an external "build Python" for all modules.
 PYTHON_FOR_FREEZE=@PYTHON_FOR_FREEZE@
@@ -700,7 +703,7 @@ clinic: check-clean-src $(srcdir)/Modules/_blake2/blake2s_impl.c
 $(BUILDPYTHON):	Programs/python.o $(LINK_PYTHON_DEPS)
 	$(LINKCC) $(PY_CORE_LDFLAGS) $(LINKFORSHARED) -o $@ Programs/python.o $(LINK_PYTHON_OBJS) $(LIBS) $(MODLIBS) $(SYSLIBS)
 
-platform: $(BUILDPYTHON) pybuilddir.txt
+platform: $(PYTHON_FOR_BUILD_DEPS) pybuilddir.txt
 	$(RUNSHARED) $(PYTHON_FOR_BUILD) -c 'import sys ; from sysconfig import get_platform ; print("%s-%d.%d" % (get_platform(), *sys.version_info[:2]))' >platform
 
 # Create build directory and generate the sysconfig build-time data there.
@@ -710,7 +713,7 @@ platform: $(BUILDPYTHON) pybuilddir.txt
 # problems by creating a dummy pybuilddir.txt just to allow interpreter
 # initialization to succeed.  It will be overwritten by generate-posix-vars
 # or removed in case of failure.
-pybuilddir.txt: $(BUILDPYTHON)
+pybuilddir.txt: $(PYTHON_FOR_BUILD_DEPS)
 	@echo "none" > ./pybuilddir.txt
 	$(RUNSHARED) $(PYTHON_FOR_BUILD) -S -m sysconfig --generate-posix-vars ;\
 	if test $$? -ne 0 ; then \
@@ -729,7 +732,7 @@ $(srcdir)/Modules/_blake2/blake2s_impl.c: $(srcdir)/Modules/_blake2/blake2b_impl
 # -s, --silent or --quiet is always the first char.
 # Under BSD make, MAKEFLAGS might be " -s -v x=y".
 # Ignore macros passed by GNU make, passed after --
-sharedmods: $(BUILDPYTHON) pybuilddir.txt @LIBMPDEC_INTERNAL@ @LIBEXPAT_INTERNAL@
+sharedmods: $(PYTHON_FOR_BUILD_DEPS) pybuilddir.txt @LIBMPDEC_INTERNAL@ @LIBEXPAT_INTERNAL@
 	@case "`echo X $$MAKEFLAGS | sed 's/^X //;s/ -- .*//'`" in \
 	    *\ -s*|s*) quiet="-q";; \
 	    *) quiet="";; \

configure.ac

@@ -143,19 +143,22 @@ AS_VAR_IF([cross_compiling], [yes],
     FREEZE_MODULE_BOOTSTRAP_DEPS='$(srcdir)/Programs/_freeze_module.py'
     FREEZE_MODULE='$(FREEZE_MODULE_BOOTSTRAP)'
     FREEZE_MODULE_DEPS='$(FREEZE_MODULE_BOOTSTRAP_DEPS)'
+    PYTHON_FOR_BUILD_DEPS=''
   ],
   [
     dnl internal build tools also depend on Programs/_freeze_module and _bootstrap_python.
     FREEZE_MODULE_BOOTSTRAP='./Programs/_freeze_module'
     FREEZE_MODULE_BOOTSTRAP_DEPS="Programs/_freeze_module"
     FREEZE_MODULE='$(PYTHON_FOR_FREEZE) $(srcdir)/Programs/_freeze_module.py'
     FREEZE_MODULE_DEPS="_bootstrap_python \$(srcdir)/Programs/_freeze_module.py"
+    PYTHON_FOR_BUILD_DEPS='$(BUILDPYTHON)'
   ]
 )
 AC_SUBST([FREEZE_MODULE_BOOTSTRAP])
 AC_SUBST([FREEZE_MODULE_BOOTSTRAP_DEPS])
 AC_SUBST([FREEZE_MODULE])
 AC_SUBST([FREEZE_MODULE_DEPS])
+AC_SUBST([PYTHON_FOR_BUILD_DEPS])
 
 AC_CHECK_PROGS([PYTHON_FOR_REGEN],
   [python$PACKAGE_VERSION python3.10 python3.9 python3.8 python3.7 python3.6 python3 python],