Fix the Meson CMake target for includedir or datadir outside the prefix

nlohmann_jsonTargets.cmake always prepended the prefix computed from the
file's location to includedir. With an absolute includedir outside the
prefix (as Nix passes for packages with a separate dev output), this gave
"<prefix>//abs/include"; with an absolute datadir outside the prefix, the
number of ".." was derived from the absolute path and the prefix resolved
to "/". In both cases find_package(nlohmann_json) failed with "Imported
target includes non-existent path". Use the absolute include directory in
these cases, as CMake's install(EXPORT) does, and check it in CI.

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

cmake/nlohmann_jsonTargets.cmake.in

@@ -4,7 +4,8 @@
 # do that, but as the library is header-only, the target only needs an include
 # 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).
+# the installation can be relocated (e.g., into a sysroot), unless includedir or
+# datadir is outside the prefix.
 
 if(TARGET @PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@)
     return()
@@ -14,7 +15,7 @@ get_filename_component(_IMPORT_PREFIX "${CMAKE_CURRENT_LIST_DIR}/@NLOHMANN_JSON_
 
 add_library(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ INTERFACE IMPORTED)
 set_target_properties(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ PROPERTIES
-    INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/@NLOHMANN_JSON_INCLUDE_INSTALL_DIR@"
+    INTERFACE_INCLUDE_DIRECTORIES "@NLOHMANN_JSON_INCLUDE_DIR@"
 )
 if(CMAKE_VERSION VERSION_LESS 3.8)
     set_target_properties(@PROJECT_NAME@::@NLOHMANN_JSON_TARGET_NAME@ PROPERTIES