Each R package should include ../../math/R/Makefile.extension, which also
defines MASTER_SITES. Consequently, it is redundant for the individual
packages to do the same. Package-specific definitions also prevent
redefining MASTER_SITES in a single common place.
sqldf is an R package for running SQL statements on R data frames,
optimized for convenience. sqldf works with the SQLite, H2,
PostgreSQL or MySQL databases.