Build SQLite extension on windows

In v.db.update there is following option:

   sqliteextra=name
   Name of SQLite extension file for extra functions (SQLite backend only)

For build SQLite extension for winGRASS follow these steps:

• Install the OSGeo4W directory structure as a build environment by these instructions
• Download extension-functions.c from here and save the file in a folder of your OSGeo4W build environment (e.g. C:\OSGeo4Wdev\src\sqliteextension)
• Download the SQLite source from here, unzip the archive and copy sqlite3.h and sqlite3ext.h in the folder of the step before (e.g. C:\OSGeo4Wdev\src\sqliteextension)
• Open the MSYS Shell offered by your OSGeo4W build environment and change to the SQLite extension directory by e.g. cd "C:\OSGeo4Wdev\src\sqliteextension" (double quotes are needed in the MSYS Shell)