* Use std.Build.Cache.Directory instead of a string for storing the
cache roots and build roots.
* Set up a std.Build.Cache in build_runner.zig and use it in
std.Build.RunStep for avoiding redundant work.
Breaking API change to std.Build.addConfigHeader. It now uses an options
struct.
Introduce std.Build.CompileStep.installConfigHeader which also accepts
an options struct. This is used to add a generated config file into the
set of installed header files for a particular compilation artifact.
std.Build.ConfigHeaderStep now additionally supports a "blank" style
where a header is generated from scratch. It no longer exposes
`output_dir`. Instead it exposes a FileSource via `output_file`.
It now additionally accepts an `include_path` option which affects the
include path of CompileStep when using the `#include` directive, as well
as affecting the default installation subdirectory for header
installation purposes.
The hash used for the directory to store the generated config file now
includes the contents of the generated file. This fixes possible race
conditions when generating multiple header files simultaneously.
The values hash table is now an array hash map, to preserve order for
the "blank" use case.
I also took the opportunity to remove output_dir from TranslateCStep and
WriteFileStep. This is technically a breaking change, but it was always
naughty to access these fields.
Usage of `catch unreachable` in build scripts is completely harmless
because build scripts are always run in Debug mode, however, it sets a
poor example for beginners to learn from.
