cppdefault.c If CROSS_DIRECTORY_STRUCTURE is defined, don't use the native
system header dir; use CROSS_INCLUDE_DIR instead if it is defined.

This just makes GCC's behavior match the documentation for the
"--with-sysroot" configure option, which corresponds to
TARGET_SYSTEM_ROOT.  The documentation says that if you specify
directories with --with-sysroot and --with-native-system-header-dir,
then the compilter will concatenate the the two together (with the
sysroot coming first) and search that directory instead of the default
/usr/include.

The concatenation is done with this line in configure.ac:

 CROSS_SYSTEM_HEADER_DIR='$(TARGET_SYSTEM_ROOT)$${sysroot_headers_suffix}$(NATIVE_SYSTEM_HEADER_DIR)'

Then Makefile.in sets the preprocessor macro CROSS_INCLUDE_DIR equal to
CROSS_SYSTEM_HEADER_DIR.

This patch reverts one of the changes from Daniel Jacobowitz on 2013-02-13.
https://github.com/gcc-mirror/gcc/commit/17acc97af91fbd116659301b0b7d4965ecc1631d

--- gcc-5.4.0/gcc/cppdefault.c
+++ gcc-5.4.0/gcc/cppdefault.c
@@ -28,9 +28,9 @@
 #define NATIVE_SYSTEM_HEADER_COMPONENT 0
 #endif
 
-#if defined (CROSS_DIRECTORY_STRUCTURE) && !defined (TARGET_SYSTEM_ROOT)
+#if defined (CROSS_DIRECTORY_STRUCTURE)
 # undef LOCAL_INCLUDE_DIR
 # undef NATIVE_SYSTEM_HEADER_DIR
 #else
 # undef CROSS_INCLUDE_DIR
 #endif