bug#27076: Cross-build support for cmake-build-system

[Ricardo Wurmus]

Hi Guix,

the following patch adds support for cross-compilation to the CMake
build system.

I tested this with the following command:

    ./pre-inst-env guix build --fallback --target=arm-linux-gnueabihf bamtools

The “bamtools” package uses the cmake-build-system and has only zlib as
an input, so it was easy to test.  Unfortunately, I do not have an arm
machine to actually test the binary, but I have no reason to assume that
the binaries won’t work because I trust that our cross toolchain does
the right thing.

--
Ricardo

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