fixincludes ignores sysroot

[Didier Kryn <kryn@xxxxxxxx>]

    Hello.

    I'm compiling gcc-6.3.0 in a chroot where there are no system 
headers in /usr/include. This means that both the toolchain used for 
build and the built compiler are sysrooted.

    The compilation fails in libmpx/mpxrt/mpxrt-utils.c because 
PATH_MAX isn't defined.

    PATH_MAX isn't defined because gcc/include-fixed/limits.h doesn't 
contain the instruction #include_next <limits.h>

    The reason why this instruction isn't there is that some process in 
fixincludes hasn't found /usr/include/limits.h. Touching this file is 
enough to solve the issue.

    It seems to me that, when the toolchain is sysrooted, fixincludes 
should search the sysroot for system headers, rather than the canonical 
location. Dunno how to achieve that though.

    Thanks.

                            Didier