unix: always build and use host Python when building 3.11

See inline comment.

I'm running into a lot of pain with `_bootstrap_python` and
`Programs/_freeze_module`. CPython's build system makes a lot
of assumptions that certain modules / symbols won't be in the
bootstrap Python. We invalidate that by statically linking
everything.

The path of least resistance is to build and use a host Python for
freezing as it avoids building the half-complete binaries altogether.

Author: indygreg

cpython-unix/build-cpython.sh

@@ -65,7 +65,10 @@ rm -rf pip-tmp
 
 # If we are cross-compiling, we need to build a host Python to use during
 # the build.
-if [ -n "${CROSS_COMPILING}" ]; then
+#
+# We also build a host Python for 3.11+ to avoid complexity with building a
+# bootstrap Python during regular build.
+if [[ -n "${CROSS_COMPILING}" || -n "${PYTHON_MEETS_MINIMUM_VERSION_3_11}" ]]; then
   pushd "Python-${PYTHON_VERSION}"
 
   # Same patch as below. See comment there.
@@ -159,6 +162,14 @@ if [ "${CC}" = "clang" ]; then
     fi
 fi
 
+# Python 3.11 supports using a provided Python to use during bootstrapping
+# (e.g. freezing). Normally it only uses this Python during cross-compiling.
+# This patch forces always using it. See comment related to
+# `--with-build-python` for more.
+if [ -n "${PYTHON_MEETS_MINIMUM_VERSION_3_11}" ]; then
+    patch -p1 -i ${ROOT}/patch-always-build-python-for-freeze.patch
+fi
+
 # Add a make target to write the PYTHON_FOR_BUILD variable so we can
 # invoke the host Python on our own.
 patch -p1 -i ${ROOT}/patch-write-python-for-build.patch
@@ -347,6 +358,13 @@ if [ -n "${CPYTHON_LTO}" ]; then
     CONFIGURE_FLAGS="${CONFIGURE_FLAGS} --with-lto"
 fi
 
+# Python 3.11 introduces a --with-build-python to denote the host Python.
+# It is required when cross-compiling. But we always build a host Python
+# to avoid complexity with the bootstrap Python binary.
+if [ -n "${PYTHON_MEETS_MINIMUM_VERSION_3_11}" ]; then
+    CONFIGURE_FLAGS="${CONFIGURE_FLAGS} --with-build-python=${TOOLS_PATH}/pyhost/bin/python${PYTHON_MAJMIN_VERSION}"
+fi
+
 if [ "${PYBUILD_PLATFORM}" = "macos" ]; then
     # Configure may detect libintl from non-system sources, such
     # as Homebrew or MacPorts. So nerf the check to prevent this.
@@ -429,11 +447,6 @@ else
 fi
 
 if [ -n "${CROSS_COMPILING}" ]; then
-    # Python 3.11 require a --with-build-python to denote the host Python.
-    if [ -n "${PYTHON_MEETS_MINIMUM_VERSION_3_11}" ]; then
-        CONFIGURE_FLAGS="${CONFIGURE_FLAGS} --with-build-python=${TOOLS_PATH}/pyhost/bin/python${PYTHON_MAJMIN_VERSION}"
-    fi
-
     # configure doesn't like a handful of scenarios when cross-compiling.
     #
     # getaddrinfo buggy test fails for some reason. So we short-circuit it.

cpython-unix/patch-always-build-python-for-freeze.patch

@@ -0,0 +1,36 @@
+diff --git a/configure.ac b/configure.ac
+index c62a565eb6..5a5a076f06 100644
+--- a/configure.ac
++++ b/configure.ac
+@@ -178,24 +178,13 @@ AC_MSG_CHECKING([for Python interpreter freezing])
+ AC_MSG_RESULT([$PYTHON_FOR_FREEZE])
+ AC_SUBST([PYTHON_FOR_FREEZE])
+ 
+-AS_VAR_IF([cross_compiling], [yes],
+-  [
+-    dnl external build Python, freezing depends on Programs/_freeze_module.py
+-    FREEZE_MODULE_BOOTSTRAP='$(PYTHON_FOR_FREEZE) $(srcdir)/Programs/_freeze_module.py'
+-    FREEZE_MODULE_BOOTSTRAP_DEPS='$(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)'
+-  ]
+-)
++dnl external build Python, freezing depends on Programs/_freeze_module.py
++FREEZE_MODULE_BOOTSTRAP='$(PYTHON_FOR_FREEZE) $(srcdir)/Programs/_freeze_module.py'
++FREEZE_MODULE_BOOTSTRAP_DEPS='$(srcdir)/Programs/_freeze_module.py'
++FREEZE_MODULE='$(FREEZE_MODULE_BOOTSTRAP)'
++FREEZE_MODULE_DEPS='$(FREEZE_MODULE_BOOTSTRAP_DEPS)'
++PYTHON_FOR_BUILD_DEPS=''
++
+ AC_SUBST([FREEZE_MODULE_BOOTSTRAP])
+ AC_SUBST([FREEZE_MODULE_BOOTSTRAP_DEPS])
+ AC_SUBST([FREEZE_MODULE])