There are two ways to build SQLite: With the Autotools setup or the
so-called "amalgamation" which is a single large C file containing the
SQLite implementation. The amalgamation build is controlled by
pre-processor flags and the Spack setup was using an amalgamation
pre-processor flag for a feature that is controlled by an option of the
configure script. As a consequence, until now Spack has always built
SQLite with the rtree feature enabled.
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