Move initialize_interpreter_pre_pyconfig() into the detail namespace.

Move the `PYBIND11_PYCONFIG_SUPPORT_PY_VERSION_HEX` define down to where it is used for the first time, and check if it is defined already, so that it is possible to customize from the compilation command line, just in case there is some unforeseen issue for Python 3.8, 3.9, 3.10.

Author: rwgk

include/pybind11/embed.h

@@ -55,7 +55,6 @@
         PYBIND11_TOSTRING(name), PYBIND11_CONCAT(pybind11_init_impl_, name));                     \
     void PYBIND11_CONCAT(pybind11_init_, name)(::pybind11::module_                                \
                                                & variable) // NOLINT(bugprone-macro-parentheses)
-#define PYBIND11_PYCONFIG_SUPPORT_PY_VERSION_HEX (0x03080000)
 
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 PYBIND11_NAMESPACE_BEGIN(detail)
@@ -93,37 +92,11 @@ inline void precheck_interpreter() {
     }
 }
 
-PYBIND11_NAMESPACE_END(detail)
+#if !defined(PYBIND11_PYCONFIG_SUPPORT_PY_VERSION_HEX)
+#    define PYBIND11_PYCONFIG_SUPPORT_PY_VERSION_HEX (0x03080000)
+#endif
 
-#if PY_VERSION_HEX >= PYBIND11_PYCONFIG_SUPPORT_PY_VERSION_HEX
-inline void initialize_interpreter(PyConfig *config,
-                                   int argc = 0,
-                                   const char *const *argv = nullptr,
-                                   bool add_program_dir_to_path = true) {
-    detail::precheck_interpreter();
-    PyStatus status = PyConfig_SetBytesArgv(config, argc, const_cast<char *const *>(argv));
-    if (PyStatus_Exception(status) != 0) {
-        // A failure here indicates a character-encoding failure or the python
-        // interpreter out of memory. Give up.
-        PyConfig_Clear(config);
-        throw std::runtime_error(PyStatus_IsError(status) != 0 ? status.err_msg
-                                                               : "Failed to prepare CPython");
-    }
-    status = Py_InitializeFromConfig(config);
-    if (PyStatus_Exception(status) != 0) {
-        PyConfig_Clear(config);
-        throw std::runtime_error(PyStatus_IsError(status) != 0 ? status.err_msg
-                                                               : "Failed to init CPython");
-    }
-    if (add_program_dir_to_path) {
-        PyRun_SimpleString("import sys, os.path; "
-                           "sys.path.insert(0, "
-                           "os.path.abspath(os.path.dirname(sys.argv[0])) "
-                           "if sys.argv and os.path.exists(sys.argv[0]) else '')");
-    }
-    PyConfig_Clear(config);
-}
-#else
+#if PY_VERSION_HEX < PYBIND11_PYCONFIG_SUPPORT_PY_VERSION_HEX
 inline void initialize_interpreter_pre_pyconfig(bool init_signal_handlers,
                                                 int argc,
                                                 const char *const *argv,
@@ -162,6 +135,38 @@ inline void initialize_interpreter_pre_pyconfig(bool init_signal_handlers,
 }
 #endif
 
+PYBIND11_NAMESPACE_END(detail)
+
+#if PY_VERSION_HEX >= PYBIND11_PYCONFIG_SUPPORT_PY_VERSION_HEX
+inline void initialize_interpreter(PyConfig *config,
+                                   int argc = 0,
+                                   const char *const *argv = nullptr,
+                                   bool add_program_dir_to_path = true) {
+    detail::precheck_interpreter();
+    PyStatus status = PyConfig_SetBytesArgv(config, argc, const_cast<char *const *>(argv));
+    if (PyStatus_Exception(status) != 0) {
+        // A failure here indicates a character-encoding failure or the python
+        // interpreter out of memory. Give up.
+        PyConfig_Clear(config);
+        throw std::runtime_error(PyStatus_IsError(status) != 0 ? status.err_msg
+                                                               : "Failed to prepare CPython");
+    }
+    status = Py_InitializeFromConfig(config);
+    if (PyStatus_Exception(status) != 0) {
+        PyConfig_Clear(config);
+        throw std::runtime_error(PyStatus_IsError(status) != 0 ? status.err_msg
+                                                               : "Failed to init CPython");
+    }
+    if (add_program_dir_to_path) {
+        PyRun_SimpleString("import sys, os.path; "
+                           "sys.path.insert(0, "
+                           "os.path.abspath(os.path.dirname(sys.argv[0])) "
+                           "if sys.argv and os.path.exists(sys.argv[0]) else '')");
+    }
+    PyConfig_Clear(config);
+}
+#endif
+
 /** \rst
     Initialize the Python interpreter. No other pybind11 or CPython API functions can be
     called before this is done; with the exception of `PYBIND11_EMBEDDED_MODULE`. The
@@ -186,7 +191,8 @@ inline void initialize_interpreter(bool init_signal_handlers = true,
                                    const char *const *argv = nullptr,
                                    bool add_program_dir_to_path = true) {
 #if PY_VERSION_HEX < PYBIND11_PYCONFIG_SUPPORT_PY_VERSION_HEX
-    initialize_interpreter_pre_pyconfig(init_signal_handlers, argc, argv, add_program_dir_to_path);
+    detail::initialize_interpreter_pre_pyconfig(
+        init_signal_handlers, argc, argv, add_program_dir_to_path);
 #else
     PyConfig config;
     PyConfig_InitIsolatedConfig(&config);