This fixes resolution of DT_NEEDED entries in cross-built libraries.
For instance, if ltdl.so needs libdl.so and has it in its RUNPATH, then
libdl.so is searched for in the right place.
* gnu/packages/cross-base.scm (cross-binutils): Pass
`--with-sysroot=/no-such-path'.
* guix/build-system/gnu.scm (inputs-search-paths): New procedure.
(standard-search-paths): Use it.
(expand-inputs): New procedure.
(standard-inputs): Use it.
(standard-cross-packages, standard-cross-inputs,
standard-cross-search-paths, gnu-cross-build): New procedures.
(gnu-build-system): Set `cross-build' field to `gnu-cross-build'.
* gnu/packages/cross-base.scm: Export `cross-gcc', `cross-binutils', and
`cross-libc'.
* guix/build/gnu-cross-build.scm: New file.
* Makefile.am (MODULES): Add it.
* gnu/packages/cross-base.scm: New file.
* gnu/packages/patches/gcc-cross-environment-variables.patch: New file.
* Makefile.am (MODULES): Add cross-base.scm.
(dist_patch_DATA): Add gcc-cross-environment-variables.patch.
* gnu/packages/base.scm (gcc-4.7): Use `LDFLAGS_FOR_TARGET' instead of
`LDFLAGS_FOR_BUILD', and use `-B' instead of `-L'.
* gnu/packages/bootstrap.scm (glibc-dynamic-linker): Add case for
"mips64el-linux".
Ludovic Courtès