Fixed imagick ext build error

This fix error when build:

```
CMake Error at CMake/EXTFunctions.cmake:20 (list):
  list sub-command REMOVE_AT requires list to be present.
Call Stack (most recent call first):
  hphp/runtime/ext/imagick/config.cmake:6 (HHVM_EXTENSION)
  hphp/runtime/ext/CMakeLists.txt:24 (include)


CMake Error at CMake/EXTFunctions.cmake:8 (list):
  list sub-command REMOVE_AT requires list to be present.
Call Stack (most recent call first):
  hphp/runtime/ext/imagick/config.cmake:12 (HHVM_LINK_LIBRARIES)
  hphp/runtime/ext/CMakeLists.txt:24 (include)
```

It also simplify cmake macro/functions using ARGN instead of ARGV