[Rd] No Rmath.h when making just standalone library

[Jussi Piitulainen]

When making only the standalone math library, include/Rmath.h
is not there, so compilation fails. A solution is to go to
src/include/ and just say `make Rmath.h' there first.

This is with R-2.2.0 on GNU/Linux on i686. The failing steps
are:

tar xzf ../R-2.2.0.tar.gz
./configure
cd src/nmath/standalone
make

Would be nice if this worked, or if the issue were just
mentioned in src/nmath/standalone/README.