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.
gsubfn is like gsub but can take a replacement function or certain
other objects instead of the replacement string. Matches and back
references are input to the replacement function and replaced by
the function output.