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.