c++17 requirement breaks compatibility for gcc7

[d.healey]

I've been compiling my projects on the oldest version of gcc that HISE could be built on in order to support as many distros as possible.

Today I went to build on my usual gcc7 but ran into an error relating to bit not being found. A little googling and this is part of c++17 which required gcc9.

Maybe now is the time to move to gcc9?

[griffinboy]

@d-healey

I keep accidentally trying to use metaprogramming concepts from c++20 and have to remember Hise is not c++20