gdlib.pc: use Requires instead of Libs when possible

[DimStar77]

gdlib.pc adds a list of libraries to add the linker command lines, which seems a bit 'strange'.

Wouldn't it be more natural to add the required dependencies to the .pc Requires: field?

[DimStar77]

(a positive side effect of this would be for downstream package maintainers to gain the feature of automatic dependency generation based on the .pc file )

[vapier]

should be possible. i don't think the distro/dep graph part is that useful in practice.