docs: Add documentation on pkgconfig.relocatable

- Documentation for the pkgconfig.relocatable module option in
Builtin-options. Gives an explanation on what it does, usefulness and
what error that can occur when using it.

- Add pkgconfig.relocatable release snippet. Similar to the
documentation in Builtin-options. Just a bit more brief.

- Add Pkgconfig to DataTests.test_builtin_options_documented in the
docs unit tests.

1 files changed, 18 insertions, 0 deletions

diff --git a/docs/markdown/snippets/pkgconfig-relocatable.md b/docs/markdown/snippets/pkgconfig-relocatable.md
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+## Installed pkgconfig files can now be relocatable
+
+The pkgconfig module now has a module option `pkgconfig.relocatable`.
+When set to `true`, the pkgconfig files generated will have their
+`prefix` variable set to be relative to their `install_dir`.
+
+For example to enable it from the command line run:
+
+```sh
+meson setup builddir -Dpkgconfig.relocatable=true …
+```
+
+It will only work if the `install_dir` for the generated pkgconfig
+files are located inside the install prefix of the package. Not doing
+so will cause an error.
+
+This should be useful on Windows or any other platform where
+relocatable packages are desired.