Detecting compilation under R

So the problem is that you needed rather

#include <R.h>
#ifdef USING_R
x = rand_unif(0.0,1.0);
#else
#include <gsl_random.h>
x = gsl_runif(0.0,1.0);
#endif

since if R.h is not around, the include will not include it.

If R.h is not around, the preprocessor will throw a tantrum:

cc    -c -o simple.o simple.c
simple.c:2:15: error: R.h: No such file or directory

- all of which would be solved if SHLIB and BUILD set a preprocessor flag to 
indicate compilation by R.

I dont see what use USING_R is. If you #include <R.h> you get it, but then 
you know you're USING_R because you included R.h, hence USING_R is going to 
always be defined in that code (unless the inclusion of R.h is conditional on 
something else...).

Oh but of course I've missed something... Just tell me what...