Move R-specific interface code from main library to R interface repo

[imciner2]

The merging of osqp/osqp#396 in the main OSQP library repo will allow us to move the R-specific printing functions to the R repo, removing the need to include the R headers and code in the main library.

This is also possible to do with the memory allocation functions currently, and hopefully the timing/interrupt functions will move to this as well, so we can then move all R-interface specific code to the interface and remove it from the main library repo - allowing us to then remove the R CMake parts from the main library and only have it here in the interface.

[imciner2]

The R-specific code for the printing functions is:

# include <R_ext/Print.h>

# define c_print Rprintf
    /* Some CRAN builds complain about __VA_ARGS__, so just print */
    /* out the error messages on R without the __FUNCTION__ trace */
# undefine c_eprint
# define c_eprint Rprintf

This should be placed into a header file, and then the OSQP_CUSTOM_PRINTING CMake option should be set to the header file containing that code.

[bnaras]

@imciner2 As I only started looking into this now, it is not clear to me which commit the above code snippet refers to. But I am happy to make any change.

One note, however: Lines 385-394 are unnecessary because CRAN procedures indicate that such details (e.g. CFLAGS, CPPFLAGS) should be set in environment variables to CMAKE. Indeed, in the currently released version 0.6.3, I just comment them out.