Do not modify source directory during build

[wxmerkt]

This separates out two commits from #269. No need to merge them now, but to keep open for inclusion in next minor release.

[ahornung]

Here's the related issue and previous discussion: #133

[mwoehlke-kitware]

ninja: error: '/tmp/octomap-src/octovis/src/extern/QGLViewer/libQGLViewer.so', needed by 'octovis/lib/liboctovis.so.1.9.1', missing and no known rule to make it

...?

[mwoehlke-kitware]

Ugh. Okay, for testing, I'd deleted source tree artifacts. Apparently, that gets built at configure time? Yuck. It's also still in the source tree:

$ git status --ignored
On branch wxm-do-not-modify-source
Ignored files:
  (use "git add -f <file>..." to include in what will be committed)
        octovis/src/extern/QGLViewer/.moc/
        octovis/src/extern/QGLViewer/.obj/
        octovis/src/extern/QGLViewer/Makefile
        octovis/src/extern/QGLViewer/libQGLViewer.prl
        octovis/src/extern/QGLViewer/libQGLViewer.so
        octovis/src/extern/QGLViewer/libQGLViewer.so.2
        octovis/src/extern/QGLViewer/libQGLViewer.so.2.6
        octovis/src/extern/QGLViewer/libQGLViewer.so.2.6.3
        octovis/src/extern/QGLViewer/ui_ImageInterface.h
        octovis/src/extern/QGLViewer/ui_VRenderInterface.h

...but at least things seem better with this PR than without...

[wxmerkt]

Hi @mwoehlke-kitware,
I forgot to add that you need to clean the source directory with git clean -fdx to test the changes. This PR is a first step but not yet complete: Ideally, the included QGLViewer gets either built to the build directory when required (e.g., as an ExternalProject) at build time or removed completely now that it appears to be available as binaries on most distributions?

Looking forward to get your expertise and insights on next steps as well :-).

Best wishes,
Wolfgang

[ahornung]

Yes, the inclusion / build of the bundled QGLViewer is still quite messy. Currently the Find... script tries to find or build it (related issue: #225).

Nowadays, an ExternalProjct would be the best solution, which was not available back then. Due to the common availability of QGLViewer, we could also just remove the bundled version.

[mwoehlke-kitware]

I'm not sure even ExternalProject is necessary... the pattern I usually see is to have an option whether to use the internal version or not. (A failed attempt to find an external version might set the default value of this.) If using the internal version, just add it as another target to the build.

Roughly¹:

find_package(QGLViewer)
if(QGLViewer_FOUND)
  set(_use_qglviewer_default OFF)
else()
  set(_use_qglviewer_default OFF)
endif()
option(OCTOMAP_USE_BUNDLED_QGLVIEWER "..." ${_use_qglviewer_default})
if(OCTOMAP_USE_BUNDLED_QGLVIEWER)
  set(QGLViewer_LIBRARIES QGLViewer)
  add_subdirectory(thirdparty/QGLViewer)
else()
  find_package(QGLViewer REQUIRED)
endif()

It's actually very difficult to (correctly) use external projects to build dependencies unless you use the "superbuild" pattern.

(¹ This is off the top of my head without checking the sources, and should be considered pseudo-code. In particular, variable names may be wrong.)