Support custom installation paths for R

[glin]

• Add an optional R_INSTALL_PATH environment variable to allow the R installation path to be changed from the default /opt/R/${R_VERSION}
• Start on new user-facing documentation for building R from source locally

These changes are meant to address two types of support issues we get with the precompiled R binaries:

• Some customers want to change the R installation path, but there's no easy way to relocate an R install without sketchy hacks. The only way to change the R install path is to install R from source with a custom --prefix.
• The Install R from Source docs are now outdated and no longer match how we install R here. The install steps for each platform have gotten very complicated, so we'll likely begin pointing users to this repo to install R from source when they need custom configuration like a different install path.

[gaborcsardi]

The installation path is embedded into a couple of scripts, and unfortunately also into the Rscript binary:

root@7ef3a7a70a1e:/opt/R/devel# rg --binary /opt/R/devel
bin/Rscript: binary file matches (found "\0" byte around offset 7)

bin/R
4:R_HOME_DIR=/opt/R/devel/lib/R
5:if test "${R_HOME_DIR}" = "/opt/R/devel/lib/R"; then
19:     if [ -x "/opt/R/devel/${libnn}/R/bin/exec/R" ]; then
20:        R_HOME_DIR="/opt/R/devel/${libnn}/R"
21:     elif [ -x "/opt/R/devel/${libnn_fallback}/R/bin/exec/R" ]; then
22:        R_HOME_DIR="/opt/R/devel/${libnn_fallback}/R"
35:R_SHARE_DIR=/opt/R/devel/lib/R/share
37:R_INCLUDE_DIR=/opt/R/devel/lib/R/include
39:R_DOC_DIR=/opt/R/devel/lib/R/doc

lib/pkgconfig/libR.pc
1:rhome=/opt/R/devel/lib/R
3:rincludedir=/opt/R/devel/lib/R/include

lib/R/bin/R
4:R_HOME_DIR=/opt/R/devel/lib/R
5:if test "${R_HOME_DIR}" = "/opt/R/devel/lib/R"; then
19:     if [ -x "/opt/R/devel/${libnn}/R/bin/exec/R" ]; then
20:        R_HOME_DIR="/opt/R/devel/${libnn}/R"
21:     elif [ -x "/opt/R/devel/${libnn_fallback}/R/bin/exec/R" ]; then
22:        R_HOME_DIR="/opt/R/devel/${libnn_fallback}/R"
35:R_SHARE_DIR=/opt/R/devel/lib/R/share
37:R_INCLUDE_DIR=/opt/R/devel/lib/R/include
39:R_DOC_DIR=/opt/R/devel/lib/R/doc

lib/R/bin/Rscript: binary file matches (found "\0" byte around offset 7)

lib/R/etc/Makeconf
7:# configure  '--prefix=/opt/R/devel' '--enable-R-shlib' '--with-tcltk' '--enable-memory-profiling' '--with-x' '--with-blas' '--with-lapack' 'R_PRINTCMD=/usr/bin/lpr' 'R_PAPERSIZE=letter' 'R_BROWSER=xdg-open' 'R_PDFVIEWER=xdg-open' 'LIBnn=lib'

So relocating would mean that we need to patch the scripts, and also the binary, which is tricky. It is possible to make the path shorter in the binary, so if we make the path very long at compilation time, then we can patch the binary as well. (I haven't actually tried this in practice.)