Set PYTHONHOME when embedding Python on Windows

Without this, the default Python path is not populated correctly,
and is based on the path to the user's executable instead.

site_scons/buildutils.py

@@ -1297,6 +1297,7 @@ def setup_python_env(env):
     env["py_version_nodot"] = py_version_nodot
     env["py_version_short"] = info["py_version_short"]
     env["py_plat"] = plat
+    env["py_base"] = info["installed_base"]
     env["site_packages"] = info["site_packages"]
     env["user_site_packages"] = info["user_site_packages"]
     if env["OS"] != "Windows":

src/SConscript

@@ -87,6 +87,9 @@ if env["python_package"] == "full":
         localenv.Depends(cythonized, pxd)
 
     obj = pyenv.SharedObject(cythonized[0])
+    if env["OS"] == "Windows":
+        escaped_home = '\\"' + pyenv["py_base"].replace("\\", "\\\\") + '\\"'
+        pyenv.Append(CPPDEFINES={"CT_PYTHONHOME": escaped_home})
     env.Command('#src/extensions/pythonExtensions.h', '#build/src/extensions/pythonExtensions.h',
                 Copy('$TARGET', '$SOURCE'))
     libraryTargets.append(obj)

src/extensions/PythonExtensionManager.cpp

@@ -11,6 +11,10 @@
 
 #include <boost/algorithm/string.hpp>
 
+#ifdef _WIN32
+#include <windows.h>
+#endif
+
 namespace ba = boost::algorithm;
 using namespace std;
 
@@ -66,6 +70,14 @@ namespace Cantera
 PythonExtensionManager::PythonExtensionManager()
 {
     if (!Py_IsInitialized()) {
+        #if defined(CT_PYTHONHOME) && defined(_WIN32)
+            const char* old_pythonhome = getenv("PYTHONHOME");
+            if (old_pythonhome == nullptr || old_pythonhome[0] == '\0') {
+                std::string pythonhome = "PYTHONHOME=";
+                pythonhome += CT_PYTHONHOME;
+                _putenv(pythonhome.c_str());
+            }
+        #endif
         Py_Initialize();
     }