Some clean up of R spkg (#13443) needs review

[Jeroen Demeyer]

I have made various fixes to the R spkg, to be reviewed at
http://trac.sagemath.org/sage_trac/ticket/13443


=== r-2.14.0.p6 (Jeroen Demeyer, 10 September 2012) ===
* #13443: some clean up of spkg-install.
* Compile R with optimization (at -O2 level).
* No longer pass -I and -L options to gcc (see #13348).
* No longer use sage_fortran (see #13349).
* Re-enable parallel make install, which works now (see #13428).