[cmake] Propagate the LLVM_VERSION cmake option to the way we find cl…

…ang. (#673)

We can specify cmake -DLLVM_VERSION=... and before that patch we will pick up
the latest version of clang which made our setup incompatible. This patch tries
harder to find the corresponding clang version to the specified version of llvm.

CMakeLists.txt

@@ -92,18 +92,15 @@ if( CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR )
   ## Find supported Clang
 
   if (DEFINED CLANG_VERSION)
-    if (CLANG_VERSION VERSION_GREATER_EQUAL CLANG_VERSION_UPPER_BOUND)
-      set(CLANG_VERSION ${CLANG_VERSION_UPPER_BOUND})
-    endif()
-    if (CLANG_VERSION VERSION_LESS CLANG_MIN_SUPPORTED)
-      set(CLANG_VERSION ${CLANG_MIN_SUPPORTED})
-    endif()
-
-    if (DEFINED Clang_DIR)
-       set(extra_hints HINTS ${Clang_DIR} "${Clang_DIR}/lib/cmake/clang" "${Clang_DIR}/cmake")
-    endif()
-
-    find_package(Clang ${CLANG_VERSION} REQUIRED CONFIG ${extra_hints})
+    message(FATAL_ERROR "Clang's configuration is not versioned. Please set LLVM_VERSION instead.")
+  endif(DEFINED CLANG_VERSION)
+
+  # We have specified -DLLVM_VERSION and we must find matching clang. However,
+  # ClangConfig.cmake is not versioned and we will need to work harder to find
+  # the correct package.
+  if (DEFINED LLVM_VERSION AND NOT DEFINED Clang_DIR)
+    set(extra_hints HINTS ${Clang_DIR} "${LLVM_BINARY_DIR}/lib/cmake/clang" "${LLVM_BINARY_DIR}/cmake")
+    find_package(Clang REQUIRED CONFIG ${extra_hints} NO_DEFAULT_PATH)
   endif()
 
   if (NOT Clang_FOUND AND DEFINED Clang_DIR)