add typealiases for exact C-compatible types

some more typealias's for consideration in the list that I forgot initially (in decreasing order of my interest level of actually having them added):
```julia
typealias Cfloat Float32
typealias Cdouble Float64
typealias Cstring Ptr{Uint8}
typealias Cargv Ptr{Ptr{Uint8}}
typealias Cptr Ptr{Void}
```

(travis build fails because I'm using OS_NAME before build_h.jl got included in the sysimg. I think it should be moved much earlier)