During running cmake -DCMAKE_INSTALL_PREFIX=/usr .. 
I have seen the message -- Could not find libclang version 3.8 or greater.
But I have LLVM together with CLANG and COMPILER-RT installed.
Please see ls output:
ved7 [ /sources/KDE/extra-cmake-modules-5.31.0/build ]$ ls /usr/lib/libclang*
/usr/lib/libclangAnalysis.a     /usr/lib/libclangDriver.a              /usr/lib/libclangIndex.a            /usr/lib/libclangSerialization.a           /usr/lib/libclangTooling.a
/usr/lib/libclangARCMigrate.a   /usr/lib/libclangDynamicASTMatchers.a  /usr/lib/libclangLex.a              /usr/lib/libclang.so                       /usr/lib/libclangToolingCore.a
/usr/lib/libclangAST.a          /usr/lib/libclangEdit.a                /usr/lib/libclangParse.a            /usr/lib/libclang.so.3.9
/usr/lib/libclangASTMatchers.a  /usr/lib/libclangFormat.a              /usr/lib/libclangRewrite.a          /usr/lib/libclangStaticAnalyzerCheckers.a
/usr/lib/libclangBasic.a        /usr/lib/libclangFrontend.a            /usr/lib/libclangRewriteFrontend.a  /usr/lib/libclangStaticAnalyzerCore.a
/usr/lib/libclangCodeGen.a      /usr/lib/libclangFrontendTool.a        /usr/lib/libclangSema.a             /usr/lib/libclangStaticAnalyzerFrontend.a