* build.zig: let user decide how to set build mode

This should delegate the responsibility of calling `standardReleaseOptions` and setting the build mode of the `*LibExeObjStep` step to the caller, especially since this might not be the process by which one wants to determine the build mode.

Also changes hides `getSrcDir` to enforce usage of `srcdir`, and asserts that the file is in fact inside a directory.

* build.zig: set root_src param to `null`

Supplying the header file as the root source here appears to cause a linker warning of the form:
```
LLD Link... warning(link): unexpected LLD stderr:
ld.lld: warning: {build_root}/zig-cache/o/{hash}/libraylib.a: archive member '{build_root}/zig-cache/o/{hash}/raylib.o' is neither ET_REL nor LLVM bitcode
```
Passing `null` instead fixes it.

zig `master` now enforces to use addIncludePath instead of addIncludeDir

We can figure out the source file location based on the location of the
build.zig file. No need to require the library user to specify where
raylib is stored.

- Adds a new OS clause to build.zig for the BSDs
- Tested on my FreeBSD box using https://github.com/Not-Nik/raylib-zig
- All demos build and render just fine.  Nice !

The build file builds Raylib as a static library for either
Windows or Linux.