alternate way to grab 'long long' type

[kevinushey]

This PR is another attempt to grab 64-bit integer types in C++98 mode in a way that won't trigger pedantic compiler warnings. Fortunately, seems to work on my Windows box using the latest Rtools toolchain.

[kevinushey]

For context, the Rtools toolchain has a c++config.h file with the contents:

/* Define if int64_t is available in <stdint.h>. */
#define _GLIBCXX_HAVE_INT64_T 1

/* Define if int64_t is a long. */
/* #undef _GLIBCXX_HAVE_INT64_T_LONG */

/* Define if int64_t is a long long. */
#define _GLIBCXX_HAVE_INT64_T_LONG_LONG 1

which I'm making use of in choosing the int64_t and uint64_t types as 'aliases' for long long and unsigned long long.

[kevinushey]

This also makes me think there might be a chance that we could sneak in long long support on other platforms with C++98. I feel like venturing there will open some old scars but it sure would sure be nice to just have long long everywhere.

[eddelbuettel]

That is a bit of a "riskier" approach as it touches the old part. Oh I see you just mod'ed that.

[kevinushey]

I think this should be ready of merge. Just to summarize the changes:

1. We explicitly enable long long and unsigned long long when the compiler reports it's compiling in C++11 (or above) mode. (Previously I think this was caught implicitly based on whether C++0x extensions were enabled, which might change in the future)

2. We also enable long long for older gcc compilers where long long is just an alias is int64_t (which is true with the Windows toolchain, currently). Fortunately, glibc reports this to us with the _GLIBCXX_HAVE_INT64_T_LONG_LONG define;

3. Finally, we allow the old logic wherein long long is brought in if we detect C++0x extensions are enabled (which appears to be true for clang by default, and for some flavors of older gcc)

I think this effectively lets us use long long on 64bit Windows (and other 64bit platforms) in a CRAN-compliant way, even if C++11 support has not been explicitly enabled.

If we find out this doesn't work as expected or we end up with warnings on some other platform we can of course back it out, but thus far things look good on my Windows VM, macOS machine and Ubuntu 16.04 VM (with gcc-5).

[eddelbuettel]

This is good stuff, and I had it go over a rev.dep run as well.

[kevinushey]

Thanks!

[eddelbuettel]

Shall we call it a day and ship this now as Rcpp 0.12.16?

[kevinushey]

Sounds good to me!