cmake: fix searching Clang RT

luzer module requires linking with a library
`clang_rt.fuzzer_no_main-x86_64` that is a part of a Clang runtime.
Without linking with it Lua runtime will report an error right on
loading `luzer.so`:

lua5.1: error loading module 'luzer' from file './luzer.so':
        ./luzer.so: undefined symbol: __sanitizer_cov_8bit_counters_init

The patch adds a module that composes a path to Clang runtime libraries
and adds this path to a library search paths.

CMakeLists.txt

@@ -5,9 +5,11 @@ project(luzer
   VERSION "1.0.0"
 )
 
+set(CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
+include(SetClangRTLibDir)
+
 find_package(Lua 5.1 REQUIRED)
 find_package(LLVM REQUIRED CONFIG)
-find_library(LIBRT rt)
 
 set(LUA_NAME "lua${LUA_VERSION_MAJOR}.${LUA_VERSION_MINOR}")
 find_program(LUA_EXECUTABLE "${LUA_NAME}")

cmake/SetClangRTLibDir.cmake

@@ -0,0 +1,25 @@
+# The function sets the given variable in a parent scope to a
+# value in CLANG_RT_LIB_DIR environment variable if it
+# is set. Otherwise the value with path to a directory with
+# Clang RT libraries is composed manually. Function raises
+# a fatal message if C compiler is not Clang.
+
+function(SetClangRTLibDir outvar)
+  set(ClangRTLibDir $ENV{CLANG_RT_LIB_DIR})
+  if (NOT CMAKE_C_COMPILER_ID STREQUAL "Clang")
+    message(FATAL_ERROR "C compiler is not a Clang")
+  endif ()
+  if (NOT ClangRTLibDir)
+    string(REPLACE "." ";" VERSION_LIST ${CMAKE_C_COMPILER_VERSION})
+    list(GET VERSION_LIST 0 CLANG_VERSION_MAJOR)
+    # Clang >= 15: /usr/lib/llvm-xxx/lib/clang/X/lib/linux/
+    # Clang <  15: /usr/lib/llvm-xxx/lib/clang/X.Y.Z/lib/linux/
+    set(CLANG_VERSION ${CMAKE_C_COMPILER_VERSION})
+    if (CLANG_VERSION_MAJOR GREATER 15)
+      set(CLANG_VERSION ${CLANG_VERSION_MAJOR})
+    endif ()
+    set(ClangRTLibDir "/usr/lib/llvm-${CLANG_VERSION_MAJOR}/lib/clang/${CLANG_VERSION}/lib/linux/")
+  endif ()
+  set(${outvar} ${ClangRTLibDir} PARENT_SCOPE)
+  message(STATUS "[SetClangRTLibDir] ${outvar} is ${ClangRTLibDir}")
+endfunction()

luzer/CMakeLists.txt

@@ -1,17 +1,4 @@
-# Locate compiler-rt libraries.
-# Location is LLVM_LIBRARY_DIRS/clang/<version>/lib/<OS>/,
-# for example LLVM_LIBRARY_DIRS/clang/4.0.0/lib/darwin/.
-#
-# See https://llvm.org/docs/LibFuzzer.html#using-libfuzzer-as-a-library
-
-set(LLVM_BASE ${LLVM_LIBRARY_DIRS}/clang/${LLVM_PACKAGE_VERSION})
-string(TOLOWER ${CMAKE_HOST_SYSTEM_NAME} OS_NAME)
-set(LIBCLANG_RT ${LLVM_BASE}/lib/${OS_NAME}/libclang_rt.fuzzer_no_main-x86_64.a)
-if(EXISTS ${LIBCLANG_RT})
-  message(STATUS "Found libclang_rt ${LIBCLANG_RT}")
-else()
-  message(FATAL_ERROR "libclang_rt is not found")
-endif()
+SetClangRTLibDir(ClangRTLibraryDir)
 
 configure_file(
   ${CMAKE_CURRENT_SOURCE_DIR}/version.c
@@ -29,11 +16,12 @@ add_library(${CMAKE_PROJECT_NAME} SHARED ${LUZER_SOURCES})
 target_include_directories(${CMAKE_PROJECT_NAME} PRIVATE
 	${LUA_INCLUDE_DIR}
 )
+target_link_directories(${CMAKE_PROJECT_NAME} PRIVATE
+                        ${ClangRTLibraryDir})
 target_link_libraries(${CMAKE_PROJECT_NAME} PRIVATE
 	${LUA_LIBRARIES}
-	${LIBRT}
-	${LIBCLANG_RT}
 	-fsanitize=fuzzer-no-link
+	clang_rt.fuzzer_no_main-x86_64
 )
 target_compile_options(${CMAKE_PROJECT_NAME} PRIVATE
 	-D_FORTIFY_SOURCE=2