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...

Barry