Determining if the __GLIBC__ exist failed with the following output:
Change Dir: /usr/ports/audio/elisa/work/.build/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/local/bin/ninja cmTC_7989d && [1/2] Building CXX object CMakeFiles/cmTC_7989d.dir/CheckSymbolExists.cxx.o
FAILED: CMakeFiles/cmTC_7989d.dir/CheckSymbolExists.cxx.o
/usr/bin/c++    -O2 -pipe -fstack-protector-strong -fno-strict-aliasing    -std=gnu++17 -o CMakeFiles/cmTC_7989d.dir/CheckSymbolExists.cxx.o -c CheckSymbolExists.cxx
CheckSymbolExists.cxx:8:19: error: use of undeclared identifier '__GLIBC__'
  return ((int*)(&__GLIBC__))[argc];
                  ^
1 error generated.
ninja: build stopped: subcommand failed.


File /usr/ports/audio/elisa/work/.build/CMakeFiles/CMakeTmp/CheckSymbolExists.cxx:
/* */
#include <stdlib.h>

int main(int argc, char** argv)
{
  (void)argv;
#ifndef __GLIBC__
  return ((int*)(&__GLIBC__))[argc];
#else
  (void)argc;
  return 0;
#endif
}