These variables get set to /usr/lib and /usr/include , which is totally wrong in a cross compilation
environment.

Changed to standard pkgconfig values consistent with most other ones.

To reserve space for the type traits sharing the same name as the ones
from standard library.

string_view_metadata was introduced to make compiled format relocatable.
However, format string compilation is an optimization and therefore
adding overhead and extra complexity for relocation is undesirable.
In most cases the string will be either static or outlive compilation
and formatting and if it doesn't, it's possible to make compiled
representation relocatale by other means.