Complete the Meson options and match CMake's compile definitions

Finish the Meson changes from #4452:

- Offer all eight options that change the CMake target, named like the
  CMake options without the JSON_ prefix. MultipleHeaders defaults to
  false so that existing Meson users keep the single header.
- Add a compile definition only for an option that differs from its
  default, exactly as target_compile_definitions does. Before, the
  GlobalUDLs and ImplicitConversions definitions were inverted and were
  also added in the default configuration.
- Keep the nlohmann_json_multiple_headers dependency that earlier
  versions of meson.build provided.
- Put the definitions into the pkg-config file and the Meson-generated
  nlohmann_jsonTargets.cmake, and install the pkg-config file into
  <datadir>/pkgconfig like CMake does.
- Ship meson_options.txt in include.zip, since meson.build now reads
  options and the zip is documented as a Meson subproject.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

cmake/nlohmann_jsonTargets.cmake.in

@@ -2,8 +2,9 @@
 #
 # CMake installations generate this file with install(EXPORT ...). Meson cannot
 # do that, but as the library is header-only, the target only needs an include
-# directory and the C++ standard. Paths are computed relative to this file so
-# that the installation can be relocated (e.g., into a sysroot).
+# directory, the C++ standard, and the compile definitions of the options that
+# differ from their defaults. Paths are computed relative to this file so that
+# the installation can be relocated (e.g., into a sysroot).
 
 if(TARGET @PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@)
     return()
@@ -25,4 +26,12 @@ else()
     )
 endif()
 
+set(_NLOHMANN_JSON_COMPILE_DEFINITIONS "@NLOHMANN_JSON_COMPILE_DEFINITIONS@")
+if(_NLOHMANN_JSON_COMPILE_DEFINITIONS)
+    set_target_properties(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ PROPERTIES
+        INTERFACE_COMPILE_DEFINITIONS "${_NLOHMANN_JSON_COMPILE_DEFINITIONS}"
+    )
+endif()
+
+unset(_NLOHMANN_JSON_COMPILE_DEFINITIONS)
 unset(_IMPORT_PREFIX)