Can't hardcode the compiler. Please specify `CXX=#{ENV.cxx}`, which will use Homebrew's selected C++ compiler. (If qcl can't build with all compilers, it's better to do selection via `fails_with`.)

Also, could you break up the string into multiple arguments? e.g.:

```ruby
system "make", "CXX=#{ENV.cxx}", "QCLDIR=#{prefix}/qcllib", "PLOPT=", "PLLIB=" 
```