Position independent code

[kjelljorner]

I came across an issue with statically linking stdlib as a dependency for a shared library target. In the end, I solved it by setting the POSITION_INDEPENDENT_CODE property for the fortran_stdlib target:

set_target_properties(fortran_stdlib PROPERTIES POSITION_INDEPENDENT_CODE ON)

One idea would be to always set this property, but as I understand this could also lead to performance issues.

[awvwgk]

I don't see an issue with setting this property by default for stdlib. Feel free to open a PR to propose such change.