Detect the R version of profiling targets

[atheriel]

At the moment we use the headers from the system version of R, and assume it is the version we sample from. This is likely false in the presence of Docker images. Moreover, there are occasional changes in the R structures that we walk that we need to detect at some point.

Unfortunately, R does not seem to have a symbol corresponding to the version in either R.bin or libR.so. An alternative is to use symbols introduced in various versions of R to guess the version indirectly.