Make regular usage of CMAKE_INSTALL_LIBDIR and GNUInstallDirs

[Zopolis4]

Use CMake's logic to define CMAKE_INSTALL_LIBDIR rather than relying on a user override, and make use of the additional abstraction that the GNUInstallDirs module allows.

[zdenop]

What is a benefit of this? Especially for non GNU installation like Windows?

[Zopolis4]

This doesn't affect a windows installation, as it only modifies unix installation code.

[zdenop]

But what it the benefit of this PR? What kind of problem are you solving?

[Zopolis4]

Avoiding the need to manually define CMAKE_INSTALL_LIBDIR as lib64 on systems where this is the default value.

[egorpugin]

Maybe change the default to lib64?
What are systems with lib today?

[stweil]

Maybe change the default to lib64?
What are systems with lib today?

Linux installations for i686, older Raspberry PI distributions, other 32 bit embedded systems don't have lib64 as far as I know.

[Zopolis4]

Not all distros use lib64-- using GNUInstallDirs ensures that tesseract will match whatever is the default for that platform.

[zdenop]

I checked openSUSE and it use booth (lib & lib64)

[zdenop]

I make basic build&install test on Windows and openSUSE and I do not see problem with this PR.

• leptonica using GNUInstallDirs already, so I do not expect problems.