Doxygen Build failing for 1.8.20 on ubuntu with Clang - clang

Question

These are the step I follow to install doxygen 1.8.20:
apt-get install graphviz
apt-get install bison
apt-get install flex
wget http://ftp.gnu.org/pub/gnu/libiconv/libiconv-1.16.tar.gz
tar -xvzf libiconv-1.16.tar.gz
cd libiconv-1.16
./configure --prefix=/usr/local/libiconv
make
make install
wget http://doxygen.nl/files/doxygen-1.8.20.src.tar.gz
gunzip doxygen-1.8.20.src.tar.gz
tar -xf doxygen-1.8.20.src.tar
cd doxygen-1.8.20
mkdir build
cd build
cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/usr -Duse_libclang=ON ..
At this point I get the following output:
CMake Error at CMakeLists.txt:51 (find_package):
Could not find a package configuration file provided by "LLVM" with any of
the following names:
LLVMConfig.cmake
llvm-config.cmake
Add the installation prefix of "LLVM" to CMAKE_PREFIX_PATH or set
"LLVM_DIR" to a directory containing one of the above files. If "LLVM"
provides a separate development package or SDK, be sure it has been
installed.
-- Configuring incomplete, errors occurred!
See also "/home/doxygen-1.8.20/build/CMakeFiles/CMakeOutput.log".
Now I get LLVM package:
apt-get install llvm
cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/usr -Duse_libclang=ON ..
At this point I get the following output:
CMake Error at CMakeLists.txt:52 (find_package):
Could not find a package configuration file provided by "Clang" with any of
the following names:
ClangConfig.cmake
clang-config.cmake
Add the installation prefix of "Clang" to CMAKE_PREFIX_PATH or set
"Clang_DIR" to a directory containing one of the above files. If "Clang"
provides a separate development package or SDK, be sure it has been
installed.
-- Configuring incomplete, errors occurred!
Get clang package:
apt-get install clang
Get the same error:
CMake Error at CMakeLists.txt:52 (find_package):
Could not find a package configuration file provided by "Clang" with any of
the following names:
ClangConfig.cmake
clang-config.cmake
Add the installation prefix of "Clang" to CMAKE_PREFIX_PATH or set
"Clang_DIR" to a directory containing one of the above files. If "Clang"
provides a separate development package or SDK, be sure it has been
installed.
-- Configuring incomplete, errors occurred!
See also "/home/doxygen-1.8.20/build/CMakeFiles/CMakeOutput.log".
Now What?

Answer 1

This was found to be a problem with the llvm and clang that get installed with apt-get install llvm and apt-get install clang. This installs version 6 of both.
Instead of these commands, do the following:
apt install llvm-10 clang-10 libclang-10-dev
Then running
cmake -G "Unix Makefiles" -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/usr -Duse_libclang=ON ..
will produce successful output:
-- Found PythonInterp: /usr/bin/python (found version "2.7.17")
-- Found FLEX: /usr/bin/flex (found version "2.6.4")
-- Found BISON: /usr/bin/bison (found version "3.0.4")
-- Looking for pthread.h
-- Looking for pthread.h - found
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD
-- Performing Test CMAKE_HAVE_LIBC_PTHREAD - Failed
-- Looking for pthread_create in pthreads
-- Looking for pthread_create in pthreads - not found
-- Looking for pthread_create in pthread
-- Looking for pthread_create in pthread - found
-- Found Threads: TRUE
-- Looking for iconv_open
-- Looking for iconv_open - found
-- Performing Test ICONV_COMPILES
-- Performing Test ICONV_COMPILES - Success
-- Found Iconv: In glibc
-- One (and only one) of the ICONV_ACCEPTS_... tests must pass
-- Performing Test ICONV_ACCEPTS_NONCONST_INPUT
-- Performing Test ICONV_ACCEPTS_NONCONST_INPUT - Success
-- Performing Test ICONV_ACCEPTS_CONST_INPUT
-- Performing Test ICONV_ACCEPTS_CONST_INPUT - Failed
-- The javacc executable not found, using existing files
-- Configuring done
-- Generating done
-- Build files have been written to: /home/libiconv-1.16/doxygen-1.8.20/build
and proceed with make and make install