GCC_INCLUDE_DIR means the same thing for native and cross. It is
where GCC stores its private include files, and also where GCC
stores the fixed include files. A cross compiled GCC runs
fixincludes on the header files in $(tooldir)/include.
(If the cross compilation header files need to be fixed, they must be
installed before GCC is built. If the cross compilation header files
are already suitable for GCC, nothing special need be done).
GPLUSPLUS_INCLUDE_DIR means the same thing for native and cross. It
is where g++ looks first for header files. The C++ library
installs only target independent header files in that directory.
LOCAL_INCLUDE_DIR is used only by native compilers. GCC
doesn't install anything there. It is normally
/usr/local/include. This is where local additions to a packaged
system should place header files.
CROSS_INCLUDE_DIR is used only by cross compilers. GCC
doesn't install anything there.
TOOL_INCLUDE_DIR is used for both native and cross compilers. It
is the place for other packages to install header files that GCC will
use. For a cross-compiler, this is the equivalent of
/usr/include. When you build a cross-compiler,
fixincludes processes any header files in this directory.