CMake flat install

[Gold856]

What's the point of the flat install option when building with CMake? They've have been broken for a while, and I'm curious as to what it used to do and if it can be removed now.

For posterity, it appears 8f2e34c broke flat installs when wpilib_dest was set to an empty string. The empty string breaks flat install since no valid path is specified to put config files into.

[calcmogul]

I don't know why we have a separate install option, because the default CMake install behavior is fine. That's what the AUR package uses: https://aur.archlinux.org/cgit/aur.git/tree/PKGBUILD?h=wpimath

[PeterJohnson]

The original idea was to make it install all the headers into one spot instead of nested under the individual library names, to avoid having to specify a lot of include paths in a Makefile build. It does feel like perhaps this has outlived its usefulness and we should just tell people to use cmake for those use cases?

[Gold856]

Flat install has been removed by #5944