only enable link what you use on GNU compilers

Clang doesn't support this flag, cause an error on osx:

> ld: unknown option: --no-as-needed

This commit should be cherry-picked to `r1.2` branch.

Author: njzjz

source/CMakeLists.txt

@@ -1,6 +1,8 @@
 cmake_minimum_required(VERSION 3.7)
 project(DeePMD)
-set(CMAKE_LINK_WHAT_YOU_USE TRUE)
+if (CMAKE_COMPILER_IS_GNU)
+  set(CMAKE_LINK_WHAT_YOU_USE TRUE)
+endif ()
 
 # build cpp or python interfaces
 if (NOT DEFINED BUILD_CPP_IF)