Fix warnings in sqlite.c in the MS-Windows build

Fix warnings in sqlite.c in the MS-Windows build

* src/sqlite.c (sqlite3_errstr) [WINDOWSNT]: Define and load from
the DLL only if the SQLite3 version is at least 3.7.15.
(sqlite_prepare_errdata) [SQLITE_VERSION_NUMBER >= 3007015]: Use
the original code if sqlite3_errstr is available.