0.5.21:

This patch release follows important enhancements in slibtool's library
interfaces and API contract. In particular, slibtool now no longer uses
streams, but rather pure file descriptors. In addition, i/o operations
now robustly handle interruption by signals, and thus do not depend on
the underlying system calls being restartable. Last but not least,
slibtool now installs a pkgconfig configuration file when applicable.

- argv.h: output usage and errors using [argv_]dprintf rather than fprint.
- argv.h: argv_get(): be consistent between forward decl. and impl.
- argv.h: move definition of struct argv_meta_impl to the implementation block.
- internals: added slbt_mapped_readline().
- internals: added slbt_map_file(), slbt_unmap_file().
- internals: added slbt_dprintf(), a signal-resilient dprintf implementation.
- driver: argv usage and error output: use the signal-resilient slbt_dprintf().
- driver: support alternate fd's for input/output/error/log.
- driver: slbt_init_host_params(): propagate failure of <compiler> -dumpmachine.
- driver: slbt_init_host_params(): do not set fnative when !fdumpmachine.
- driver: struct slbt_fd_ctx: added .fdcwd and .fddst.
- library: invocations of openat() now use the context's fdcwd.
- project: added initial pkgconfig support.
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