Avoid installing libraries multiple times or issueing wrong message

Some libraries call `boost_install` themselves which adds
`INSTALL_INTERFACE` to their `INTERFACE_INCLUDE_DIRETORIES` property.
That then makes the install check in `__boost_auto_install` fail and
print a message that the library won't be installed while it will be.

Use a label to detect targets for which `boost_install` was already
called and skip the logic in `__boost_auto_install` for those.

include/BoostInstall.cmake

@@ -320,6 +320,10 @@ function(boost_install_target)
 
   install(EXPORT ${LIB}-targets DESTINATION "${CONFIG_INSTALL_DIR}" NAMESPACE Boost:: FILE ${LIB}-targets.cmake)
 
+  get_target_property(labels ${LIB} LABELS)
+  list(APPEND labels boost_install_specified)
+  set_target_properties(${LIB} PROPERTIES LABELS "${labels}")
+
   set(CONFIG_FILE_NAME "${CMAKE_CURRENT_BINARY_DIR}/tmpinst/${LIB}-config.cmake")
   set(CONFIG_FILE_CONTENTS "# Generated by BoostInstall.cmake for ${LIB}-${__VERSION}\n\n")
 

include/BoostRoot.cmake

@@ -182,6 +182,11 @@ function(__boost_auto_install __boost_lib)
 
     if(TARGET "Boost::${__boost_lib_target}" AND TARGET "boost_${__boost_lib_target}")
 
+      get_target_property(labels "boost_${__boost_lib_target}" LABELS)
+      if("boost_install_specified" IN_LIST labels)
+        return() # Ignore libraries for which boost_install was already called
+      endif()
+
       get_target_property(__boost_lib_incdir "boost_${__boost_lib_target}" INTERFACE_INCLUDE_DIRECTORIES)
 
       set(incdir "${BOOST_SUPERPROJECT_SOURCE_DIR}/libs/${__boost_lib}/include")