Remove PYTHONPATH from the environment on Windows

This make sure to use the the modules distributed with LyX,
avoiding reconfigure failures.

(cherry picked from commit ad0c9a548b)

src/support/os_win32.cpp

@@ -158,6 +158,11 @@ void init(int argc, char ** argv[])
 	 */
 
 
+	// Remove PYTHONPATH from the environment as it may point to an
+	// external python installation and cause reconfigure failures.
+	unsetEnv("PYTHONPATH");
+
+
 #if defined(_MSC_VER) && (_MSC_VER >= 1900)
 	// Removing an argument from argv leads to an assertion on Windows
 	// when compiling with MSVC 2015 in debug mode (see bug #10440).

status.23x

@@ -142,6 +142,9 @@ What's new
 
 * INTERNALS
 
+- Fix reconfigure issues on Windows in the presence of an external
+  python installation.
+
 
 
 * DOCUMENTATION AND LOCALIZATION