cmake LIBC_HAS_BACKTRACE examples

omnigres/omnigres deps/h2o/CMakeLists.txt :80

CHECK_C_SOURCE_COMPILES("
#include <execinfo.h>
int main(void) {
void *p[10];
int ret = backtrace(p, 10);
backtrace_symbols_fd(p, ret, 2);
return 0;
}" LIBC_HAS_BACKTRACE)