Assume build.conf not exists if build file found instead of directory

[Cordain]

My attempt to build the build script with cc nob.c -o build && ./build ended up with
[ERROR] Could not check if file ./build/build.conf exists: Not a directory
This encounter is odd for fresh builds, since there is no chance of having build directory anyway.

To mitigate this ambiguity, I propose an additional check if errno is ENOTDIR, so a file build will not be confused with directory of old configuration scheme.