Add -fPIC for the build system

[JerrySievert]

This adds -fPIC for the build process, which is needed for any code that needs to be position indepentent, like something that would end up linking for a shared object.

this appears to work correctly on linux and macOS, but I have not been able to confirm under windows.

[bnoordhuis]

Please ignore the review; I deleted it again. I clicked cancel because I wanted to ask a general question but GH posted it anyway 🤦

Anyway, the question: does this turn on -fPIC always or only when you pass -Denable-fPIC=ON to cmake?

[JerrySievert]

it turns on PIC always. I was struggling for a reason why any build would want PIC disabled. is there a reason why a build would want to disable it? if so, I can make it available with a flag.

[bnoordhuis]

It should be a flag. On some architectures (notably 32 bits x86 because of register pressure) position independent code is 5-10% slower than non-PIC code. It's why node didn't turn on PIE for the longest time.

[JerrySievert]

okee dokee, can do. my vacation is over, so it might take a couple of days to get the PR updated, in the meantime, I'll convert it into a draft.

[JerrySievert]

changed to -Denable-fPIC=ON - if you want me to change the if() statements to wrap around the actual enable instead of each individual, I can, but even though this is a few more lines of code I felt it was clearer.

happy to make any changes though.