Bug#1065416: requesting input on recent posts to #1065416

I struggled with the build system, and I know the real problem:

1. linux-libc-dev-ARCH-cross is useful, for some case if we maintain an cross toolchain of a port
that Debian is not supported yet.  The example is src:cross-toolchain-base-mipsen.
I support the cross toolchain for 12 MIPS ports: EL/BE X 32/64/N32 X R2/R6.


2. It is not OK to search base headers when we configure gcc itself.
    https://patchwork.sourceware.org/project/gcc/patch/[email protected]/

    GCC/configure.ac detects features by the headers of linux/glibc.
    Let’s have a example: we are building a mips64el cross toolchain on a amd64 machine:
         If gcc/configure.ac sees /usr/include/limits.h before than /usr/mips64el-linux-gnuabi64/include/limits.h
    Our mips64el-linux-gnuabi64-gcc will have some problem.

    And you can find the `fixinclude` in GCC for more information. 


@Mattias In fact. The current gcc-X-cross searches /usr/include when configure, it is dangerous.
      --includedir=/usr/mipsel-linux-gnu/include --with-sysroot=/  —prefix=/usr
Is not good.
The prefer way should be `—prefix=/usr` only  with my patch.