Fixed installation of pkg-config file on other than Ubuntu

distributions.
Signed-off-by: Vitaly Zaitsev <vitaly@easycoding.org>

CMakeLists.txt

@@ -40,6 +40,7 @@ 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")
 
 if (JSON_MultipleHeaders)
     set(NLOHMANN_JSON_INCLUDE_BUILD_DIR "${PROJECT_SOURCE_DIR}/include/")
@@ -158,6 +159,6 @@ endif()
     )
     install(
         FILES "${CMAKE_BINARY_DIR}/${PROJECT_NAME}.pc"
-        DESTINATION lib/pkgconfig
+        DESTINATION ${NLOHMANN_JSON_PKGCONFIG_INSTALL_DIR}
     )
 endif()