New comment by ericonr on void-packages repository

https://github.com/void-linux/void-packages/pull/32563#issuecomment-901346788

Comment:
```
[263/392] Building CXX object CMakeFiles/ncls.dir/src/ls/main.cpp.o
FAILED: CMakeFiles/ncls.dir/src/ls/main.cpp.o 
/builddir/.xbps-notcurses/wrappers/arm-linux-musleabihf-c++ -D_DEFAULT_SOURCE -D_FORTIFY_SOURCE=2 -D_GNU_SOURCE -D_XOPEN_SOURCE=700 -I/builddir/notcurses-2.3.15/include -I/builddir/notcurses-2.3.15/src -I/builddir/notcurses-2.3.15 -I/builddir/notcurses-2.3.15/build/include -DNDEBUG -fstack-clash-protection -D_FORTIFY_SOURCE=2 -O2 -march=armv6 -mfpu=vfp -mfloat-abi=hard   -I/usr/arm-linux-musleabihf/usr/include -fvisibility=hidden -Wall -Wextra -W -Wshadow -Wformat -Wformat-security -fexceptions -fstrict-aliasing -std=c++17 -MD -MT CMakeFiles/ncls.dir/src/ls/main.cpp.o -MF CMakeFiles/ncls.dir/src/ls/main.cpp.o.d -o CMakeFiles/ncls.dir/src/ls/main.cpp.o -c /builddir/notcurses-2.3.15/src/ls/main.cpp
In file included from /builddir/notcurses-2.3.15/include/notcurses/notcurses.h:11,
                 from /builddir/notcurses-2.3.15/include/notcurses/direct.h:4,
                 from /builddir/notcurses-2.3.15/include/ncpp/Direct.hh:5,
                 from /builddir/notcurses-2.3.15/src/ls/main.cpp:16:
/usr/arm-linux-musleabihf/usr/include/string.h:69:7: error: declaration of 'char* strndup(const char*, size_t)' has a different exception specifier
   69 | char *strndup (const char *, size_t);
      |       ^~~~~~~
In file included from /builddir/notcurses-2.3.15/src/ls/main.cpp:15:
/builddir/notcurses-2.3.15/src/compat/compat.h:82:7: note: from previous declaration 'char* strndup(const char*, size_t) noexcept'
   82 | char* strndup(const char* str, size_t size) NOEXCEPT;
      |       ^~~~~~~
[264/392] Building CXX object CMakeFiles/ncplayer.dir/src/player/play.cpp.o
ninja: build stopped: subcommand failed.
=> ERROR: notcurses-2.3.15_1: do_build: '${make_cmd} ${makejobs} ${make_build_args} ${make_build_target}' exited with 1
```

it's triggering compatibility function for `strndup` on musl, which then conflicts with the libc declaration. This is probably happening because it tests for `strndup` without defining `_GNU_SOURCE` or `_POSIX_C_SOURCE=200809L`. Should be simple to fix in the build system