Added sqlite3 driver fro tables in GMPL

Hello !

I just added a sqlite3 driver for tables in GMPL with this commit 
https://github.com/mingodad/GLPK/commit/66a2ff9ec7fd5e2506cfebceca4bab95203b9739 
, it also include the updated examples in examples/sql .

And in doing so I discovered a possible use after free in 
src/mpl/mpsql.c for "xfree(sql->query);" in the ODBC driver and 
"xfree(query);" in the MySQL driver.

To compile it (windows will need a bit more work):

====

./autogen.sh

./configure --enable-sqlite --enable-dl

====

Any comment/suggestion/help is welcome !

Cheers !