Not finding path for standard library

**Describe the bug**
Executing a command like the following:
```
hyde ./include/psutil/safe_cast.hpp -hyde-src-root=../../include -hyde-yaml-dir=../../docs/include \
-resource-dir=`clang++ --print-resource-dir` -access-filter-protected \
-namespace-blacklist="safe_cast_detail" -hyde-update -- \
-x c++ -std=c++14 -stdlib=libc++
```

Will error out trying to include any file in libc++. The workaround is to explicitly add the path:
```
-I`xcode-select --print-path`/usr/include/c++/v1
```

Start by reproducing the shown hyde command on macOS with -stdlib=libc++ and inspect how the command's resource and include paths are handled. Trace why libc++ headers are not found unless the xcode-select path is supplied manually. Done means the standard library is found without the extra -I workaround, with a regression check if the repository provides a suitable test location.

Written by the indexing model from the issue text.