Performing Test LIBCLANG_COMPILES - Failed

Hi. I know this issue has been emerged multiple times but none of them solved the problem.

I cloned the master, made a build directory and from there:

cmake -DCMAKE_CXX_COMPILER=/usr/local/bin/clang++ -DCMAKE_C_COMPILER=/usr/local/bin/clang ..

The following outcome is similar when I call the the cmake without C and CXX flags. Either way I get this

-- The C compiler identification is GNU 7.4.0
-- The CXX compiler identification is GNU 7.4.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- 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: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- llvm-config executable found: /usr/local/bin/llvm-config
-- Using Clang version 8.0.0 from /usr/local/lib with CXXFLAGS -I/usr/local/include -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS
-- Found LibClang: /usr/local/lib/libclang.so  
-- Performing Test LIBCLANG_COMPILES
-- Performing Test LIBCLANG_COMPILES - Failed
CMake Error at src/CMakeLists.txt:88 (message):
  Failed to compile small clang test app.

  It's likely that the include file <clang-c/Index.h> could not be found!

  Maybe you need to install the clang development package (delete the
  CMakeCache.txt file before trying to run cmake again after installation)?

  See CMakeFiles/CMakeError.log for more info.

-- Configuring incomplete, errors occurred!
See also "/tmp/rtags/build/CMakeFiles/CMakeOutput.log".
See also "/tmp/rtags/build/CMakeFiles/CMakeError.log".

The CMakeFiles/CMakeError.log suggests that /usr/bin/ld: cannot find -lxml2:

Performing C++ SOURCE FILE Test LIBCLANG_COMPILES failed with the following output:
Change Dir: /tmp/rtags/build/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_f1bd2/fast"
/usr/bin/make -f CMakeFiles/cmTC_f1bd2.dir/build.make CMakeFiles/cmTC_f1bd2.dir/build
make[1]: Entering directory '/tmp/rtags/build/CMakeFiles/CMakeTmp'
Building CXX object CMakeFiles/cmTC_f1bd2.dir/src.cxx.o
/usr/local/bin/clang++    -I/usr/local/include -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -DLIBCLANG_COMPILES  -I/usr/local/include -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS
Linking CXX executable cmTC_f1bd2
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_f1bd2.dir/link.txt --verbose=1
/usr/local/bin/clang++   -I/usr/local/include -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS -D__STDC_FORMAT_MACROS -D__STDC_LIMIT_MACROS -DLIBCLANG_COMPILES  -I/usr/local/include -D_GNU_SOURCE -D__STDC_CONSTANT_MACROS 
/usr/bin/ld: cannot find -lxml2
clang-8: error: linker command failed with exit code 1 (use -v to see invocation)
CMakeFiles/cmTC_f1bd2.dir/build.make:98: recipe for target 'cmTC_f1bd2' failed
make[1]: *** [cmTC_f1bd2] Error 1
make[1]: Leaving directory '/tmp/rtags/build/CMakeFiles/CMakeTmp'
Makefile:126: recipe for target 'cmTC_f1bd2/fast' failed
make: *** [cmTC_f1bd2/fast] Error 2

However, from the first cmake call, I know it uses -- llvm-config executable found: /usr/local/bin/llvm-config and:

$ /usr/local/bin/llvm-config --system-libs
-lz -lrt -ldl -ltinfo -lpthread -lm -lxml2

In other words, llvm-config knows the libraries but somehow when building the test they cannot be linked.

I have few guesses. The https://github.com/Andersbakken/rtags/issues/493#issuecomment-151894344 suggests that multiple llvm installation could cause problems and I do indeed have multiple ones:

$ dpkg --get-selections | grep -i 'libclang'
libclang-8-dev                  install
libclang-common-8-dev               install
libclang1-8                 install

$ dpkg --get-selections | grep -i 'llvm'
libllvm6.0:amd64                install
libllvm7:amd64                  install
libllvm7:i386                   install
libllvm8:amd64                  install
libllvm8:i386                   install
llvm-8                      install
llvm-8-dev                  install
llvm-8-runtime                  install

Similarly, I have two clang-c/Index.h at

/usr/local/include/clang-c/Index.h
/usr/lib/llvm-8/include/clang-c/Index.h

and two llvm-config:

/usr/local/bin/llvm-config
/usr/lib/llvm-8/bin/llvm-config

Cmake tells me it uses the first which knows about -lxml2 as well, however, calling the second one from command line with --system-libs returns nothing.

Could this be the source of the problem? If so how can I fix it? If not, what else can I try?

Thanks a lot and cheers!

Andersbakken / rtags

Performing Test LIBCLANG_COMPILES - Failed #1350