v1.0rc4 Release Candidate

This is a bugfix release and a potential 1.0 candidate.

There are no actual code changes in this release, most of the patches are to the build system. Thanks to Jan Pokorný for, er, all of them.
I've bumped the library soname to 0.18.0 which should really have happened last time.

Changes from 1.0rc3

• build: fix tests/_syslog_override.h not being distributed
• build: enable syslog tests when configuring in spec
• build: do not install syslog_override for the RPM packaging
• build: update library soname to 0.18.0
• build: do not try to second-guess "distdir" Automake variable
• build: switch to XZ tarball format for {,s}rpm packaging
• CI: make sure RPM can be built all the time
• Generating the man pages definitely doesn't depend on existence of
  (possibly generated) header files that we omit anyway.
• build: drop extra qbconfig.h rule for auto_check_header self-test
• build: extra clean-local rule instead of overriding clean-generic
• build: docs: {dependent -> public}_headers + more robust obtaining
• build: tests: grab "public_headers" akin to docs precedent
• build: fix preposterous usage of $(AM_V_GEN)
• build: tests: add intermediate check-headers target
• CI: "make check" already included in "make distcheck"
• build: fix out-of-tree build broken with 0b04ed5 (#184)
• docs: enhance qb_log_ctl* description + fix doxygen warning
• docs: apply "doxygen -u" on {html,man}.dox.in, fix deprecations
• docs: {html,man}.dox.in: strip options for unused outputs
• docs: {html,man}.dox.in: unify where reasonable
• docs: make README.markdown always point to "CURRENT" docs
• build: reorder LINT_FLAGS in a more logical way
• build: make the code splint-friendly where not already
• build: better support for splint checker
• build: make splint check tolerant of existing defects