Add CMAKE_BUILD_INTERFACE_INCLUDES build-variable.

This makes

 set(CMAKE_BUILD_INTERFACE_INCLUDES ON)

add the equivalent of

 set_property(TARGET tgt APPEND PROPERTY
   INTERFACE_INCLUDE_DIRECTORIES
   $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR};${CMAKE_CURRENT_BINARY_DIR}>
 )

to every target.

If the headers are in CMAKE_CURRENT_SOURCE_DIR, and the generated headers
are in CMAKE_CURRENT_BINARY_DIR, this is a convenient way to build a target
bar, which depends on foo, just by using target_link_libraries() and adding
the INTERFACE_INCLUDE_DIRECTORIES to the INCLUDE_DIRECTORIES of the target
being linked. There will be more-convenient porcelain API to consume the
property in the future.

Source/cmDocumentVariables.cxx

@@ -1146,6 +1146,17 @@ void cmDocumentVariables::DefineVariables(cmake* cm)
      false,
      "Variables that Control the Build");
 
+  cm->DefineProperty
+    ("CMAKE_BUILD_INTERFACE_INCLUDES", cmProperty::VARIABLE,
+     "Automatically add the current source- and build directories "
+     "to the INTERFACE_INCLUDE_DIRECTORIES.",
+     "If this variable is enabled, CMake automatically adds for each "
+     "target ${CMAKE_CURRENT_SOURCE_DIR} and ${CMAKE_CURRENT_BINARY_DIR} "
+     "to the INTERFACE_INCLUDE_DIRECTORIES."
+     "By default CMAKE_BUILD_INTERFACE_INCLUDES is OFF.",
+     false,
+     "Variables that Control the Build");
+
   cm->DefineProperty
     ("CMAKE_INSTALL_RPATH", cmProperty::VARIABLE,
      "The rpath to use for installed targets.",

Source/cmGlobalGenerator.cxx

@@ -938,6 +938,23 @@ void cmGlobalGenerator::Generate()
       (*targets)[tit->first] = tit->second;
       (*targets)[tit->first].SetMakefile(mf);
       }
+
+    for ( tit = targets->begin(); tit != targets->end(); ++ tit )
+      {
+      if (mf->IsOn("CMAKE_BUILD_INTERFACE_INCLUDES"))
+        {
+        const char *binDir = mf->GetStartOutputDirectory();
+        const char *srcDir = mf->GetStartDirectory();
+        const std::string dirs = std::string(binDir ? binDir : "")
+                                + std::string(binDir ? ";" : "")
+                                + std::string(srcDir ? srcDir : "");
+        if (!dirs.empty())
+          {
+          tit->second.AppendProperty("INTERFACE_INCLUDE_DIRECTORIES",
+                                ("$<BUILD_INTERFACE:" + dirs + ">").c_str());
+          }
+        }
+      }
     }
 
   // Add generator specific helper commands

Tests/CMakeCommands/target_link_libraries/CMakeLists.txt

@@ -53,6 +53,13 @@ set_target_properties(targetA PROPERTIES LINK_INTERFACE_LIBRARIES "")
 
 assert_property(targetA LINK_INTERFACE_LIBRARIES "")
 
+add_subdirectory(subdir)
+target_link_libraries(targetA subdirlib)
+set_property(TARGET targetA APPEND PROPERTY
+  INCLUDE_DIRECTORIES
+    $<TARGET_PROPERTY:subdirlib,INTERFACE_INCLUDE_DIRECTORIES>
+)
+
 target_link_libraries(targetA depB depC)
 
 assert_property(targetA LINK_INTERFACE_LIBRARIES "")

Tests/CMakeCommands/target_link_libraries/subdir/CMakeLists.txt

@@ -0,0 +1,5 @@
+
+set(CMAKE_BUILD_INTERFACE_INCLUDES ON)
+
+add_library(subdirlib SHARED subdirlib.cpp)
+generate_export_header(subdirlib)

Tests/CMakeCommands/target_link_libraries/subdir/subdirlib.cpp

@@ -0,0 +1,7 @@
+
+#include "subdirlib.h"
+
+int SubDirLibObject::foo() const
+{
+  return 0;
+}

Tests/CMakeCommands/target_link_libraries/subdir/subdirlib.h

@@ -0,0 +1,12 @@
+
+#ifndef SUBDIRLIB_H
+#define SUBDIRLIB_H
+
+#include "subdirlib_export.h"
+
+struct SUBDIRLIB_EXPORT SubDirLibObject
+{
+  int foo() const;
+};
+
+#endif

Tests/CMakeCommands/target_link_libraries/targetA.cpp

@@ -3,6 +3,8 @@
 #include "depC.h"
 #include "depIfaceOnly.h"
 
+#include "subdirlib.h"
+
 int main(int argc, char **argv)
 {
   DepA a;
@@ -11,5 +13,7 @@ int main(int argc, char **argv)
 
   DepIfaceOnly iface_only;
 
-  return a.foo() + b.foo() + c.foo() + iface_only.foo();
+  SubDirLibObject sd;
+
+  return a.foo() + b.foo() + c.foo() + iface_only.foo() + sd.foo();
 }