Build SQLite extension on windows

Problem: SQLite lacks by default log(), sqrt(), and other functions.

Solution: In v.db.update there is the 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)
• Then type in the MSYS Shell following command: gcc -fPIC -lm -shared extension-functions.c -o libsqlitefunctions.dll

Then libsqlitefunctions.dll is built and can be used by v.db.update (example from the manual):

   g.copy vect=precip_30ynormals,myprecip_30ynormals
   v.db.addcolumn myprecip_30ynormals column="logjuly double precision"
   v.db.update myprecip_30ynormals column="logjuly" query_column="log(jul)" \
     sqliteextra=C:\OSGeo4Wdev\src\sqliteextension\libsqlitefunctions.dll