New comment by acobaugh on void-packages repository

https://github.com/void-linux/void-packages/pull/11802#issuecomment-496316659
Comment:
There are 2 issues with cross-compiling this package:
1) wsjtx is a cmake "uber" project, with hamlib and wsjtx itself as sub-projects. I patched the parent CMakeLists.txt to set `--host` on the hamlib ./configure command. The patch also passes the toolchain file down to the wsjtx subproject
2) The toolchain file was overriding `CMAKE_FIND_ROOT_PATH`. This was preventing the wsjtx subproject from finding the previously built libraries. I've worked around this in a post_configure() by appending to the toolchain file a `list(APPEND list item)` to put the project's build directory back in.

I'm now running into an issue with qmake setting (I think) the wrong --isystem/-I, resulting in this error:
```
[ 69%] Building CXX object CMakeFiles/wsjt_qtmm.dir/Audio/BWFFile.cpp.o
In file included from /usr/aarch64-linux-gnu/usr/include/c++/8.3.0/ext/string_conversions.h:41,
                 from /usr/aarch64-linux-gnu/usr/include/c++/8.3.0/bits/basic_string.h:6400,
                 from /usr/aarch64-linux-gnu/usr/include/c++/8.3.0/string:52,
                 from /usr/aarch64-linux-gnu/usr/include/c++/8.3.0/stdexcept:39,
                 from /usr/aarch64-linux-gnu/usr/include/c++/8.3.0/array:39,
                 from /builddir/wsjtx-2.0.1/build/wsjtx-prefix/src/wsjtx/Audio/BWFFile.hpp:4,
                 from /builddir/wsjtx-2.0.1/build/wsjtx-prefix/src/wsjtx/Audio/BWFFile.cpp:1:
/usr/aarch64-linux-gnu/usr/include/c++/8.3.0/cstdlib:75:15: fatal error: stdlib.h: No such file or directory
 #include_next <stdlib.h>
               ^~~~~~~~~~
```

I'm unsure of how to fix this latest error, or even if my other fixes are "kosher" w.r.t. good packaging practices. I've grep'd through srcpkgs and I can't find any other packages that run into this issue.