Export cmake as jsoncons::jsoncons

Fix #517

https://cmake.org/cmake/help/latest/guide/importing-exporting/index.html explains what it means.

It's possible to temporarily make the old name to be alias for the new name for compatibility, if desired. Not implemented in this commit yet.

Author: DarthGandalf

CMakeLists.txt

@@ -45,6 +45,7 @@ install(TARGETS jsoncons
 
 # Makes the project importable from the build directory
 export(EXPORT ${PROJECT_NAME}-targets
+       NAMESPACE jsoncons::
        FILE "${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}Targets.cmake")
 
 install(DIRECTORY ${JSONCONS_INCLUDE_DIR}/jsoncons
@@ -75,5 +76,6 @@ install(FILES ${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}Config.cmake
         DESTINATION ${JSONCONS_CMAKECONFIG_INSTALL_DIR})
 install(EXPORT ${PROJECT_NAME}-targets
         FILE ${PROJECT_NAME}Targets.cmake
+        NAMESPACE jsoncons::
         DESTINATION ${JSONCONS_CMAKECONFIG_INSTALL_DIR})