build: install .pc and .cmake files to share/ (#3619)

As nlohmann_json is a header-only library, its pkg-config and cmake
config files should be installed to `share/` (GNUInstallDirs' DATADIR),
as `share/` is the canonical directory where architecture-independent
files should be stored, while `lib/` is for architecture-dependent stuff
(see the [FHS][]).

Apart from being technically correct, installing to `share/` can help
with cross-compiling, for example in Debian-based environments. If
you're interested, I'd suggest reading this [thread][].

Related to #1697

[FHS]: https://refspecs.linuxfoundation.org/FHS_3.0/fhs-3.0.html#usrshareArchitectureindependentData
[thread]: marzer/tomlplusplus#165 (comment)

CMakeLists.txt

@@ -57,15 +57,15 @@ endif ()
 include(GNUInstallDirs)
 
 set(NLOHMANN_JSON_TARGET_NAME               ${PROJECT_NAME})
-set(NLOHMANN_JSON_CONFIG_INSTALL_DIR        "${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME}" CACHE INTERNAL "")
+set(NLOHMANN_JSON_CONFIG_INSTALL_DIR        "${CMAKE_INSTALL_DATADIR}/cmake/${PROJECT_NAME}" CACHE INTERNAL "")
 set(NLOHMANN_JSON_INCLUDE_INSTALL_DIR       "${CMAKE_INSTALL_INCLUDEDIR}")
 set(NLOHMANN_JSON_TARGETS_EXPORT_NAME       "${PROJECT_NAME}Targets")
 set(NLOHMANN_JSON_CMAKE_CONFIG_TEMPLATE     "cmake/config.cmake.in")
 set(NLOHMANN_JSON_CMAKE_CONFIG_DIR          "${CMAKE_CURRENT_BINARY_DIR}")
 set(NLOHMANN_JSON_CMAKE_VERSION_CONFIG_FILE "${NLOHMANN_JSON_CMAKE_CONFIG_DIR}/${PROJECT_NAME}ConfigVersion.cmake")
 set(NLOHMANN_JSON_CMAKE_PROJECT_CONFIG_FILE "${NLOHMANN_JSON_CMAKE_CONFIG_DIR}/${PROJECT_NAME}Config.cmake")
 set(NLOHMANN_JSON_CMAKE_PROJECT_TARGETS_FILE "${NLOHMANN_JSON_CMAKE_CONFIG_DIR}/${PROJECT_NAME}Targets.cmake")
-set(NLOHMANN_JSON_PKGCONFIG_INSTALL_DIR     "${CMAKE_INSTALL_LIBDIR}/pkgconfig")
+set(NLOHMANN_JSON_PKGCONFIG_INSTALL_DIR     "${CMAKE_INSTALL_DATADIR}/pkgconfig")
 
 if (JSON_MultipleHeaders)
     set(NLOHMANN_JSON_INCLUDE_BUILD_DIR "${PROJECT_SOURCE_DIR}/include/")