clang doesn't support -fuse-linker-plugin

This flag '-fuse-linker-plugin' is unsupported and causes an error with
```
Apple LLVM version 5.1 (clang-503.0.40) (based on LLVM 3.4svn)
```
e.g. when compiling mariadb 10.0.12
```
clang: error: unknown argument: '-fuse-linker-plugin' [-Wunused-command-line-argument-hard-error-in-future]
clang: note: this will be a hard error (cannot be downgraded to a warning) in the future
```