Fix NaCl system header files location

Fix NaCl system header files location

NATIVE_SYSTEM_HEADER_DIR is target makefile variable that describes where system headers are located, relative to file system root. Default is /usr/include.

When building a cross-compiler configured using --with-sysroot, CROSS_SYSTEM_HEADER_DIR is constructed from sysroot value followed by NATIVE_SYSTEM_HEADER_DIR.

We don’t have /usr/include on NaCl, so set this to /include.

This only affects configurations that use --with-sysroot, so the actual effect will become visible only when I add --with-sysroot to gcc configuration.

R=pasko@google.com
BUG=http://code.google.com/p/nativeclient/issues/detail?id=854
TEST=

Committed: http://git.chromium.org/gitweb?p=native_client/nacl-gcc.git;a=commit;h=1b292e3

[pasko-google - do not use, 2011-06-15 17:43:03]

LGTM

please, also, provide explanation of:
1. why GCC cross-compiler searches sys-include
2. how NATIVE_SYSTEM_HEADER_DIR is converted to CROSS_SYSTEM_HEADER_DIR
3. GCC algorithm to search for limits.h

.. you could also clarify that with the next change you'd eliminate sys-include
to cleanup the tools/Makefile