build: fix linking against monolithic LLVM

Fixes: https://github.com/iovisor/bpftrace/issues/251

src/ast/CMakeLists.txt

@@ -36,7 +36,7 @@ if (STATIC_LINKING)
   target_link_libraries(ast ${clang_libs})
   target_link_libraries(ast ${llvm_libs})
 else()
-  find_library(found_LLVM LLVM)
+  find_library(found_LLVM LLVM HINTS ${LLVM_LIBRARY_DIRS})
   if(found_LLVM)
     target_link_libraries(ast LLVM)
   else()
@@ -44,6 +44,5 @@ else()
     llvm_expand_dependencies(llvm_libs ${_llvm_libs})
     target_link_libraries(ast ${llvm_libs})
   endif()
-
   target_link_libraries(ast libclang)
 endif()