New comment by ericonr on void-packages repository

https://github.com/void-linux/void-packages/pull/26962#issuecomment-762436152

Comment:
@yopito there were some more suggestions for some include paths, since clang apparently gets them very wrong. I have yet to try and implement them, see:

```
2021-01-17 22:00:11     q66     -isystem /usr/${XBPS_CROSS_TRIPLET}/usr/include/c++/`${XBPS_CROSS_TRIPLET}-gcc -dumpversion`
2021-01-17 22:00:40     q66     to the flags passed to clang
2021-01-17 22:01:00     ericonr does that manage to find c++config.h?
2021-01-17 22:01:08     ericonr it's in a really weird location
2021-01-17 22:01:28     ericonr like /usr/include/c++/9.3/$triplet
2021-01-17 22:02:27     q66     ah, no it won't
2021-01-17 22:02:31     q66     you'll have to add three paths
2021-01-17 22:03:11     q66     particularly /usr/triplet/usr/include/c++/<ver>, and then also its subdirs <triplet>, and backward
2021-01-17 22:03:57     q66     just add them all with -isystem
2021-01-17 22:04:09     q66     probably something like
2021-01-17 22:04:21     q66     local gcc_ver=`${XBPS_CROSS_TRIPLET}-gcc -dumpversion`
2021-01-17 22:04:26     ericonr by backward do you mean literally "backward"?
2021-01-17 22:04:34     q66     local inc_root=/usr/${XBPS_CROSS_TRIPLET}/usr/include/c++/${gcc_ver}
2021-01-17 22:04:35     q66     and then
2021-01-17 22:04:52     q66     -isystem ${inc_root} -isystem ${inc_root}/${XBPS_CROSS_TRIPLET} -isystem ${inc_root}/backward
```

Also, I believe you should still keep sysroot