Drop supporting Trusty and use imported targets for dependencies

[jslee02]

This PR is a sequel of #1209, and more will be followed to modernize CMake scripts of DART. Here are two main changes in this PR.

• Drop supporting Ubuntu 14.04 (Trusty) in DART 6.7. The main reason is to use the recent CMake features (e.g., target-based dependency management), which requires CMake >= 3. Also, this is to reduce the effort of supporting the too old version of dependencies.

• Use IMPORTED targets for dependency libraries where available. For the dependencies that don't provide targets, define the targets in the finding modules, Find<package>.cmake. Use the same target name when the corresponding dependency provides the target in their latest version, which is not released yet.

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[scpeters]

I'm having trouble building this on mojave.

First it has trouble linking to assimp:

[ 65%] Linking CXX shared library ../lib/libdart.dylib
ld: library not found for -lassimp
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [lib/libdart.6.7.0.dylib] Error 1

Then if I manually add the path to the assimp library to LIBRARY_PATH, it gets a little farther, then fails to find a framework:

make[2]: *** No rule to make target `/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk/System/Library/Frameworks/GLUT.framework/GLUT', needed by `lib/libdart-gui.6.7.0.dylib'.  Stop.
make[1]: *** [dart/gui/CMakeFiles/dart-gui.dir/all] Error 2

[scpeters]

The release-6.7 branch builds fine on my mojave machine

[jslee02]

@scpeters I think there were two issues in building on Mojave, which should be fixed by the last few commits.

First, I switched to use the upstream config file of assimp, but they use a hard-coded library path that doesn't work for macOS. So I fall back to using own module file, Findassimp.cmake.

Second, GLUT::GLUT seems not working for Mojave or maybe for all the macOSs. This is because FindGLUT.cmake, provided by CMake, uses a different library path for the link library property of GLUT::GLUT from GLUT_LIBRARIES. The link library property of GLUT::GLUT additionally has /GLUT suffix, and it causes the build error you saw. So I just had to fall back to using variables for GLUT.

[scpeters]

I just tested 7c13f5d on our mojave test machine, and it builds successfully. Nice work!

[jslee02]

Thanks for the testing! Let me merge this PR once Travis CI becomes happy.