target_include_directories: Support relative SYSTEM include dirs (#15464)

Source/cmTargetIncludeDirectoriesCommand.cxx

@@ -75,7 +75,23 @@ bool cmTargetIncludeDirectoriesCommand
   tgt->InsertInclude(this->Join(content), lfbt, prepend);
   if (system)
     {
-    tgt->AddSystemIncludeDirectories(content);
+    std::string prefix =
+      this->Makefile->GetCurrentSourceDirectory() + std::string("/");
+    std::set<std::string> sdirs;
+    for (std::vector<std::string>::const_iterator it = content.begin();
+      it != content.end(); ++it)
+      {
+      if (cmSystemTools::FileIsFullPath(it->c_str())
+          || cmGeneratorExpression::Find(*it) == 0)
+        {
+        sdirs.insert(*it);
+        }
+      else
+        {
+        sdirs.insert(prefix + *it);
+        }
+      }
+    tgt->AddSystemIncludeDirectories(sdirs);
     }
   return true;
 }
@@ -91,7 +107,7 @@ void cmTargetIncludeDirectoriesCommand
 
   if (system)
     {
-    std::string joined = cmJoin(content, ";");
+    std::string joined = this->Join(content);
     tgt->AppendProperty("INTERFACE_SYSTEM_INCLUDE_DIRECTORIES",
                         joined.c_str());
     }

Tests/IncludeDirectories/SystemIncludeDirectories/CMakeLists.txt

@@ -35,6 +35,16 @@ add_library(imported_consumer2 imported_consumer.cpp)
 target_link_libraries(imported_consumer2 imported_consumer)
 target_compile_options(imported_consumer2 PRIVATE -Werror=unused-variable)
 
+# add a target which has a relative system include
+add_library(somelib imported_consumer.cpp)
+target_include_directories(somelib SYSTEM PUBLIC "systemlib_header_only")
+target_compile_options(somelib PRIVATE -Werror=unused-variable)
+
+# add a target which consumes a relative system include
+add_library(otherlib upstream.cpp)
+target_link_libraries(otherlib PUBLIC somelib)
+target_compile_options(somelib PRIVATE -Werror=unused-variable)
+
 macro(do_try_compile error_option)
   set(TC_ARGS
     IFACE_TRY_COMPILE_${error_option}