Update to 1.6.0

Changelog:
Changes from 1.5.4 to 1.6.0:
----------------------------

* Bug fixes:
  - Fixed issue where packages which referenced missing packages in
    Requires.private may have crashed due to memory corruption issues
    in some circumstances.
  - Fixed warnings reported by GCC 8 diagnostics.

* Enhancements:
  - Add LIBPKGCONF_VERSION and LIBPKGCONF_VERSION_STR macros for
    determining libpkgconf version.
  - Add pkgconf_fragment_copy_list() to copy a fragment list to
    another fragment list.

Changes from 1.5.3 to 1.5.4:
----------------------------

* Bug fixes:
  - fix build on Windows with Meson
  - fix edge cases for path canonicalization (especially on Windows)

Changes from 1.5.2 to 1.5.3:
----------------------------

* Security fixes:
  - Fix edge cases involving dequoting zero-length tuples that can lead to a
    buffer overflow under the right circumstances.  Thanks to A. Wilcox for
    reporting and supplying a patch.  (MR 3)

Changes from 1.5.1 to 1.5.2:
----------------------------

* Bug fixes:
  - Ensure environment variables override values learned from personality files
    or built-in defaults.

* Documentation enhancements:
  - Add pkgconf-personality(5) manpage documenting the personality file format.

Changes from 1.5.0 to 1.5.1:
----------------------------

* Bug fixes:
  - fixed a crash with some invalid multi-line .pc files

Changes from 1.4.2 to 1.5.0:
---------------------------

* Administrative:
  - The git repository has moved to <https://git.dereferenced.org/pkgconf/pkgconf>,
    due to the acquisition of GitHub by Microsoft.

* Overall enhancements:
  - pkgconf now supports the proposed Requires.internal pkg-config extension,
    by merging it with the Requires.private list (there is no functional difference
    between the two in our resolver implementation)
  - Support for cross-compilation personalities have been added.  To make use of this
    functionality, create a file in the new personality.d directory that sits inside
    the pkgconfig directory.  The personality file format is described in
    pc-personality(5). (github #166)
  - Support for Haiku has been added, including interpretation of BELIBRARIES and
    other toolchain specifics. (github #180)
  - Testsuite support can be disabled when building with Meson. (github #175)

* Bug fixes:
  - tuples are now appropriately dequoted when added by the parser (github #186).

* Various Windows enhancements:
  - CMake supports building with GCC on Windows. (github #179)
  - Prefix rewriting has been improved. (github #177)
  - PKGCONF_API support has been implemented when building with Meson,
    allowing Meson to be used to build pkgconf on Windows. (github #174)

* Documentation fixes:
  - The manpages have been linted and fixed. (github #181, #182, #183)
  - The description of pkgconf --exists has been corrected. (github #173)

Changes from 1.4.1 to 1.4.2:
----------------------------

* Bug fixes:
  - ensure pkgconf_dependency_t nodes have a solution marked when satisfied
    by an indirect provider (github #172)


(ryoon)