[Rd] back port of Bug 15899 fix missing from R 3.2.1 RC release!!!

[Jack Howarth]

    Is there a reason why the fix for Bug 15899 - Omitted 'extern' on
'R_running_as_main_program' after refactor can cause linker errors for
applications embedding R...

https://bugs.r-project.org/bugzilla3/show_bug.cgi?id=15899

was never back ported for R 3.3 for the R 3.2.1 release? Restoring
that omitted 'extern' to the declaration of 'int
R_running_as_main_program;' in src/Rinterface.h is essential for being
able to build Rstudio.
           Jack