Also find clang include path based on runtime libclang library path

My KDevelop was compiled against my system libclang v7.0.1, thus
KDEV_CLANG_BUILTIN_DIR points to /usr/lib/clang/7.0.1/include. Now
I compiled clang 8 from sources and put it in a different folder
in my home path and adapted LD_LIBRARY_PATH to make KDevelop pick it
up. This then fails to find the builtin include path, since it tries
to find `/usr/lib/clang/8.0.1/include` which doesn't exit.

This patch adds another fallback to fix the above scneario: We now
lookup the path of libclang at runtime through dlfcn.h's dladdr,
which we pass a libclang function address, here clang_getClangVersion.
Then on success we deduce the include path by removing the library
name and then instead appending `clang/$version/include`, which makes
it use /home/milian/projects/compiled/other/lib/clang/8.0.1/include
as required.
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failed #3825
linux kf5-qt5 qt5.10

01:00:00

 
Name Stage Failure
failed
linux kf5-qt5 qt5.10 Build The script exceeded the maximum execution time set for the job
        Start   7: test_aggregatemodel
7/121 Test #7: test_aggregatemodel ..................... Passed 1.54 sec
Start 8: test_toolviewtoolbar
kf5.kinit.klauncher: new app ":1.3"
8/121 Test #8: test_toolviewtoolbar .................... Passed 0.95 sec
Start 9: test_projectmodel
kf5.kinit.klauncher: new app ":1.4"
Pulling docker image gitlab/gitlab-runner-helper:x86_64-fa86510e ...
ERROR: Job failed: execution took longer than 1h0m0s seconds