[BUILD] Fix LLVM static/dynamic link issue (#1461)

This patch prevents libtvm.so from both containing static parts of LLVM
and link with libLLVM.so by removing the latter from link list.

cmake/util/FindLLVM.cmake

@@ -18,7 +18,14 @@ macro(find_llvm use_llvm)
   if(LLVM_CONFIG STREQUAL "ON")
     find_package(LLVM REQUIRED CONFIG)
     llvm_map_components_to_libnames(LLVM_LIBS all)
-    list(REMOVE_ITEM LLVM_LIBS LTO)
+    list (FIND LLVM_LIBS "LLVM" _llvm_dynlib_index)
+    if (${_llvm_dynlib_index} GREATER -1)
+      set(LLVM_LIBS LLVM)
+      message(STATUS "Link with dynamic LLVM library")
+    else()
+      list(REMOVE_ITEM LLVM_LIBS LTO)
+      message(STATUS "Link with static LLVM libraries")
+    endif()
     set(TVM_LLVM_VERSION ${LLVM_VERSION_MAJOR}${LLVM_VERSION_MINOR})
   elseif(NOT LLVM_CONFIG STREQUAL "OFF")
     # use llvm config