build options C++ "java-like" (gcc)
_afaict_ on old Q6600 **not using** `-flto` is a little more than 0.1s faster.

`main-naive-java-like.cpp`

```
$ /usr/bin/g++ -O3 --std=c++17 -flto -s -o unsci_run unsci.c++
$ time ./unsci_run
331665
real	0m0.756s
user	0m0.755s
sys	0m0.000s
```

```
$ /usr/bin/g++ -O3 --std=c++17 -s -o unsci_run unsci.c++
$ time ./unsci_run
331665
real	0m0.634s
user	0m0.634s
sys	0m0.000s
```

