Sage build critical path

[Jeroen Demeyer]

Purely for information, this is the critical path for building Sage in
parallel using "make -j" (unlimited number of jobs). This is after
applying #13392 (remove GNUTLS-related packages).

prereq
bzip2
patch
termcap
readline
sqlite
python
atlas
r
rpy2

By far the most time taken is taken by R and ATLAS, with Python in a
firm third place.

With #13395 (fix upgrading), this list would become slightly longer:

prereq
bzip2
patch
termcap
readline
sqlite
python
mercurial
sage_root
lapack
atlas
r
rpy2