Fix the mips64el build (and enable optimization again)

@edd This fixes https://buildd.debian.org/status/fetch.php?pkg=quantlib-swig&arch=mips64el&ver=1.32-1&stamp=1698321785&raw=0 but it also affects all other architectures.

The root cause of this problem is that building such huge C++ code without optimization produces code that is not only slow but also huge. On amd64 this commit reduces the size of _QuantLib.cpython-311-x86_64-linux-gnu.so from 38 MB to 25 MB (and that's not even optimizing for size, which would give 21 MB). Building with default optimization makes the binary code small enough to fix the mips64el build.

Apparently there was some (toolchain?) issue with toolchains that had LTO enabled by default 4 years ago, but I was not able to reproduce that building with -flto on amd64. If any problem with LTO gets reported, adding fno-lto would be a less invasive solution.

It is possible that building with optimization might cause problems on some other architecture, for that it would be easiest if you could upload this change and I'll submit a followup change for architectures where it causes issues.