C++11 R compilation failure

With `CXX_STD = CXX11` set in src/Makevars, I get:

```
* installing to library ‘/Library/Frameworks/R.framework/Versions/3.1/Resources/library’
* installing *source* package ‘RcppAnnoy’ ...
** libs
I/Library/Frameworks/R.framework/Resources/include -DNDEBUG -I../inst/include/ -I/usr/local/include -I"/Library/Frameworks/R.framework/Versions/3.1/Resources/library/Rcpp/include" -I"/Library/Frameworks/R.framework/Versions/3.1/Resources/library/BH/include"      -c annoy.cpp -o annoy.o
/bin/sh: I/Library/Frameworks/R.framework/Resources/include: No such file or directory
make: [annoy.o] Error 127 (ignored)
-dynamiclib -Wl,-headerpad_max_install_names -undefined dynamic_lookup -single_module -multiply_defined suppress -L/Library/Frameworks/R.framework/Resources/lib -L/usr/local/lib -L/usr/local/lib -o RcppAnnoy.so annoy.o -F/Library/Frameworks/R.framework/.. -framework R -Wl,-framework -Wl,CoreFoundation
/bin/sh: -dynamiclib: command not found
make: *** [RcppAnnoy.so] Error 127
ERROR: compilation failed for package ‘RcppAnnoy’
* removing ‘/Library/Frameworks/R.framework/Versions/3.1/Resources/library/RcppAnnoy’
```

Note that the commands being generated by R aren't even sensible.