Use more GNUInstallDirs

[a17r]

Conveniently fixes multiarch install dir issues.

[clanmills]

Andreas. Thanks for this contribution. Luis (piponazo) is our CMake expert and will review your changes.

[piponazo]

Hi @a17r , thanks a lot for the contribution.

I did not know about the GNUInstallDirs module. I actually think that I might have removed something about it during the CMake refactoring it is taking place during the last weeks.

Could you please explain briefly what is your user case and why this is needed ?

I have checked this and it works perfectly. However I left a comment about the usage of CMAKE_SOURCE_INCLUDEDIR. On Windows, the value of that variable is empty, and I could not find any reference to it on the web. Might it be a typo ?

[a17r]

Thanks for catching that one, fixed the bogus var.

Basically, everyone should use GNUInstallDirs with cmake - it is giving us sane defaults to work with, less re-invented wheels, and easy to override vars for different distro needs (Gentoo has libx32, lib32, lib64, other distributions may have more complicated multiarch names). With exiv2 as-is, a 32/64 bit multiarch build would simply install twice into /usr/lib/libexiv2.so, raising an error with strict checks. And instead of chasing libdir paths we now even get automatically correct /usr/lib{32,64}/pkgconfig files.

[piponazo]

The comment I made has been addressed. Now the PR only make changes related to the GNUInstallDirs

[piponazo]

I think this might be a typo. I have been searching for this CMAKE_SOURCE_INCLUDEDIR and I did not found anything about it. Could you give any reference to its usage or double check if it is a typo ?

[piponazo]

I actually checked again and the lines 292 and 293 can be removed. The exiv2 target is propagating already the needed include directories.