[R-SIG-Win] Rtools and gfortran

[Jeroen Ooms]

For those helping with testing with the new tool chain:

Some packages containing Fortran code (such as 'vegan') which worked
with the old tool chain, gave linking errors under the new one. It
turns out that the gcc 4.9.3 requires you to explicitly link libm and
libquadmath. So in Makeconf we need:

FLIBS = -lgfortran -lm -lquadmath

The same flags are used on Linux as well. I am not a Fortran expert,
but from what I can see these libraries always get statically linked
so the compiled R binary packages should still work on machines
without a tool chain.