compilation fails with gcc-15

Error message below
```
                 from /home/sangalli/data/local-libs/ChASE-github/tests/noinput.cpp:7:
/opt/gcc/15.2.0/include/c++/15.2.0/ext/concurrence.h:257:32: error: cannot convert ‘<brace-enclosed initializer list>’ to ‘unsigned int’ in initialization
  257 |     __gthread_cond_t _M_cond = __GTHREAD_COND_INIT;
      |                                ^~~~~~~~~~~~~~~~~~~
```