Don't look for libLLVM without llvm-config or LLVM.cmake help

-- [IGC] LLVM targets are not defined. Searching for LLVM.
-- [IGC] Using system LLVM 10.0.0
CMake Error at IGC/CMakeLists.txt:3283 (message):
  [IGC] Could not find the LLVM dylib.  Aborting.

--- IGC/CMakeLists.txt.orig	2020-05-19 05:44:04 UTC
+++ IGC/CMakeLists.txt
@@ -3278,12 +3278,8 @@ if(LLVM_LINK_LLVM_DYLIB)
         set(IGC_BUILD__LLVM_LIBS_TO_LINK "${SET_LLVM_LIB_PATH}/libLLVM-${LLVM_VERSION_MAJOR}.so")
         message(STATUS "[IGC] Link against specified LLVM dylib ${IGC_BUILD__LLVM_LIBS_TO_LINK}")
     else()
-        find_library(IGC_BUILD__LLVM_LIBS_TO_LINK "libLLVM-${LLVM_VERSION_MAJOR}.so")
-        if(IGC_BUILD__LLVM_LIBS_TO_LINK)
-          message(STATUS "[IGC] Link against found LLVM dylib ${IGC_BUILD__LLVM_LIBS_TO_LINK}")
-        else()
-          message(FATAL_ERROR "[IGC] Could not find the LLVM dylib. Aborting.")
-        endif()
+        set(IGC_BUILD__LLVM_LIBS_TO_LINK "LLVM")
+        message(STATUS "[IGC] Link against LLVM dylib")
     endif()
 
 else()