Fix builds without GL

[Squareys]

Hi @mosra !

I applied your patch for the fix for building WITH_GL=OFF. If building with BUILD_DEPRECATED=ON, it would not find External/OpenGL, which I made sure that it gets installed anyway now (untested!).

Cheers, Jonathan

The issue you get when trying to build a project using magnum that was build accordingly:

-- The C compiler identification is GNU 4.8.5
-- The CXX compiler identification is GNU 4.8.5
-- Check for working C compiler using: Ninja
-- Check for working C compiler using: Ninja -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler using: Ninja
-- Check for working CXX compiler using: Ninja -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Found Corrade: /usr/local/include  found components:  Utility Containers 
-- Found Corrade: /usr/local/include  found components:  Utility Containers rc 
-- Found Magnum: /usr/local/include   
-- LIB_SUFFIX variable is not defined. It will be autodetected now.
-- You can set it manually with -DLIB_SUFFIX=<value> (64 for example)
-- LIB_SUFFIX autodetected as '64', libraries will be installed into /builds/vhiterabbit/webvr-pong/deploy/lib64
-- Found Magnum: /usr/local/include  found components:  Magnum 
-- Configuring done
CMake Error in src/server/CMakeLists.txt:
  Imported target "Magnum::Magnum" includes non-existent path

    "/usr/local/include/MagnumExternal/OpenGL"

  in its INTERFACE_INCLUDE_DIRECTORIES.  Possible reasons include:

  * The path was deleted, renamed, or moved to another location.

  * An install or uninstall procedure did not complete successfully.

  * The installation package was faulty and references files it does not
  provide.

[mosra]

After thinking about this a bit more, this should be just if(WITH_GL). TARGET_GL can be only enabled if WITH_GL is enabled.

[mosra]

In many places I used the following preprocessor statement:

#if defined(MAGNUM_BUILD_DEPRECATED) && defined(MAGNUM_TARGET_GL)

but there are apparently cases that I missed, having just #ifdef MAGNUM_BUILD_DEPRECATED. Sorry.

... so, instead of installing the GL stuff even when the user explicitly doesn't want to, the deprecated stuff depending on GL should not be compiled at all when TARGET_GL is OFF. So all these CMake patches that add GL stuff should be removed and replaced by preprocessor patches that remove GL stuff.

I have no easy way of testing this myself without creating a barebones VM from scratch, so I'm kinda depending on you to find and fix all those cases :) Thank you 👍

[Squareys]

Alright, that will require time that I currently don't have, though. I will have the time to work through this as of the 9th of July, I hope that is okay.

[mosra]

I fixed most of the issues affecting core library in 89d9883 while integrating the Vulkan Triangle example. There might be some more left, but since none of the changes of this PR would get used anyway, I'm closing it.