0.5.3:

This patch release includes a few minor fixes and several improvements, all
related to slibtool's own build system.

- build system: app.tag: avoid unconditional step execution.
- build system: configure: added --strict, --ccstrict, --ldstrict support.
- build system: Makefile: added the .conf convenience target.
- build system: Makefile: added the .display_cc convenience target.
- build system: Makefile: added the -display_cflags convenience target.
- build system: Makefile: added custom header installation support.
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