Install CMake package config files with Meson, and add Meson options

Squashed onto develop from:
- Install CMake package config files with Meson
- meson: Indent code inside an if block
- meson: set a minimum Meson version
- meson: use `override_dependency()` to set dependencies
- meson: use `install_subdir` for headers
- meson: set the C++ standard to C++11
- meson: handle single header and multiheader the same way CMake does
- meson: add support for the GlobalUDLs option
- meson: add support for the ImplictConversions option
- Add meson information to FILES.md
- Complete the Meson options and match CMake's compile definitions
- Add the JSON_* definitions to the CMake pkg-config file
- Document the Meson options and check them in CI
- Fix the Meson CMake target for includedir or datadir outside the prefix
- Avoid //include in Meson-generated CMake target for prefix /
- Add DisableTupleReferenceConversion to Meson and the CMake pkg-config file
- Check in CI that Meson and pkg-config offer the CMake options
- Remove accidentally committed Python bytecode

Co-authored-by: Dylan Baker <dylan@pnwbakers.com>
Signed-off-by: Dylan Baker <dylan@pnwbakers.com>
Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tools/check_build_options/README.md

@@ -0,0 +1,21 @@
+# check_build_options
+
+Checks that the Meson build and the pkg-config files offer the same options as the CMake target, so that a new CMake
+option is not forgotten in one of them.
+
+The compile definitions of the CMake target (`target_compile_definitions` in [`CMakeLists.txt`](../../CMakeLists.txt))
+are the reference. When you add an option there, also add it to
+
+- the pkg-config block in `CMakeLists.txt` (`NLOHMANN_JSON_PKGCONFIG_CFLAGS`),
+- [`meson_options.txt`](../../meson_options.txt), named without the `JSON_` prefix and with the same default,
+- [`meson.build`](../../meson.build) (`json_defines`), and
+- the list of Meson options in
+  [`docs/mkdocs/docs/integration/package_managers.md`](../../docs/mkdocs/docs/integration/package_managers.md).
+
+Run the check with
+
+```shell
+make check_build_options
+```
+
+It needs only Python 3 and runs in the `ci_meson_install` CI job.