Compile R with debugging flags

[wch]

Consider compiling all builds with _GLIBCXX_ASSERTIONS and possibly _GLIBCXX_DEBUG. This will help catch some C++ errors earlier on. (For example, rstudio/httpuv#133.)

https://gcc.gnu.org/onlinedocs/libstdc++/manual/using_macros.html

Note that _GLIBCXX_DEBUG, the documentation says, "this flag changes the sizes and behavior of standard class templates such as std::vector, and therefore you can only link code compiled with debug mode and code compiled without debug mode if no instantiation of a container is passed between the two translation units."
https://gcc.gnu.org/onlinedocs/libstdc++/manual/debug_mode_using.html

[wch]

I've added _GLIBCXX_DEBUG (which in turn defines _GLIBCXX_ASSERTIONS) in a16eba3.

[david-cortes]

@wch It seems the macro is not currently active in these debug builds. Here I'm attaching a sample package which should trigger an assertion error for out-of-bounds access of a vector element when running the examples. There is no assertion error when testing the example, nor when installing the package and calling the function.
assertions_1.0.tar.gz

[wch]

@david-cortes I removed those flags because of #5. It is possible that the issue has been fixed by now in R itself and we can add those flags back, but I currently don't have time to test that out. If you test it out and you run R CMD check on several packages and they don't raise those warnings, then I'd be willing to add the flags back.