Specifying compiler executable in config.nims

[vitaliy]

Thanks @exelotl, you rock! Worked flawlessly, I've just added this to 
config.nims and voila:
    
    
    put "arm64.android.clang.path", getEnv("ANDROID_NDK") & 
"/toolchains/llvm/prebuilt/darwin-x86_64/bin"
    put "arm64.android.clang.exe", "clang"
    switch("passC", "-isysroot " & getEnv("ANDROID_NDK") & "/sysroot -target 
aarch64-linux-android23")
    
    
    Run