math/R: fix compiler flags embedded within Makeconf

The R package installs the file lib/R/etc/Makeconf, which is intended
to be used by R packages that themselves compile programs.  This
feature is rarely used, but the math/R-nimble package is an example of
one that does.  For this to work, the compiler flags embedded within
Makeconf must be compatible with the system compiler.  At least on
MacOS, this is not the case by default, and so nimble compilations
fail.  This substitutes ${COMPILER_RPATH_FLAG} into configure.ac, so
that it is used when creating Makeconf.  Since neither R itself nor
most R packages compile other programs, Makeconf is generally not used
and this fix will have no impact.

(brook)