[DBD-SQLite] ok to add a Makefile.PL knob for enabling RTREE support

[George Hartzell]

I use SQLITE's Rtree code and as the Makefile.PL currently stands I
need to unpack it, add -DSQLITE_ENABLE_RTREE' to @CC_DEFINE, repack
the tarball and then install it by hand instead of just being able to
use cpan.

I'd like to propose (and I'd be happy to generate a patch) something
along the lines of adding a check for an environment variable like
DBD_SQLITE_ENABLE_RTREE and if it's present/true/... then adjust
@CC_DEFINE.

Any reason this approach won't work or is a bad idea?

Thanks,

g.