1.0 problem: is_windows(), is_linux, etc. need to be redefined in build.jl

It's a pretty easy fix, just replace e.g. `is_windows()` with `Sys.iswindows()`.

I will open a PR to fix it fairly soon. The reason I'm not doing so immediately is because I'm not sure what should be done about macOS support.

I have a vague sense that people want to move away from Homebrew.jl, so perhaps the method for installing the dependency should be changed? I also know that Apple has controversially deprecated OpenCL support in 10.14, so perhaps it may be necessary to drop support altogether.

```
┌ Error: Error building `CLBLAS`:
│ ERROR: LoadError: UndefVarError: is_windows not defined
│ Stacktrace:
│ [1] top-level scope at none:0
│ [2] include at ./boot.jl:317 [inlined]
│ [3] include_relative(::Module, ::String) at ./loading.jl:1038
│ [4] include(::Module, ::String) at ./sysimg.jl:29
│ [5] include(::String) at ./client.jl:388
│ [6] top-level scope at none:0
│ in expression starting at /Users/curry/.julia/packages/CLBLAS/U0XIf/deps/build.jl:10
```