Append to CFLAGS and CXXFLAGS to use values passed to ./configure

Append to CFLAGS and CXXFLAGS to use values passed to ./configure. This can allow users to customize the SQLite build process with compile time #defines rather than altering the sources directly.  EG, with this,

```shell
CFLAGS="-DSQLITE_MAX_COLUMN=4000" ./configure
```
Allows for compile time determination of the maximum number of columns allowed in a SQLite table.