Make mksquashfs binary location (optionally) configurable

[probonopd]

@adrianschroeter asked:

It would be nice if the mksquashfs binary location could become a configure/compile time option.

[TheAssassin]

Maybe ask back whether a CMake option would suffice?

[probonopd]

Since he mentioned "compile time option" I think yes.

[TheAssassin]

I created a PR in #618. @adrianschroeter this allows for setting the path like -DMKSQUASHFS_DIR=lib/appimagekit, relative to the install root directory (i.e., the directory which is prepended by CMAKE_INSTALL_PREFIX normally. If set, appimagetool will look for mksquashfs relative to the appimagetool binary.

@probonopd we might think about making the directory openSUSE uses the default, though. It isn't any more difficult than putting it in bin, and could be considered a "sane default" for other distros such as Nitrux, too. Also, it's a bit more explicit, since it doesn't rely on the PATH environment variable.

[TheAssassin]

@adrianschroeter this, by the way, should eliminate any maintainer patches, as the binary is also installed into the right directory, this doesn't just alter the "search path" for it.

[probonopd]

Actually /usr/share/... is for architecture-independent code only.

http://www.pathname.com/fhs/pub/fhs-2.3.html#USRSHAREARCHITECTUREINDEPENDENTDATA

This hierarchy is intended to be shareable among all architecture platforms of a given OS; thus, for example, a site with i386, Alpha, and PPC platforms might maintain a single /usr/share directory that is centrally-mounted.

So we should put it somewhere else.

[probonopd]

/usr/lib/appimagekit might possibly be a better idea:
http://www.pathname.com/fhs/pub/fhs-2.3.html#USRLIBLIBRARIESFORPROGRAMMINGANDPA

/usr/lib includes (...) internal binaries that are not intended to be executed directly by users or shell scripts.
Applications may use a single subdirectory under /usr/lib. If an application uses a subdirectory, all architecture-dependent data exclusively used by the application must be placed within that subdirectory.

Some distributions use libexec but I can't find that in FHS.

[TheAssassin]

Yes, you're right, it should be lib/appimagekit. Edited the comment.

[TheAssassin]

Closed via #618.

[probonopd]

We are now getting

No such file or directory: /usr/local/bin/../lib/appimagekit/mksquashfs

Reference:
https://travis-ci.org/probonopd/linuxdeployqt/builds/328445959#L1588

This currently breaks linuxdeployqt builds.

[TheAssassin]

This is not an issue in AppImageKit, but a linuxdeployqt one. uploadtool should be blamed, as it deleted the release although it couldn't upload files for a new one.