CMakeList: do not overwrite module path

Currently, the CMakeList.txt completely overwrites the CMAKE_MODULE_PATH
variable.

This is problematic when an upper-layer buildsystem wants to set its own
module path to use custom modules.

For example, Buldroot [0] provides a custom platform description [1] to fix
cross-compilation issue. Overwriting the module path means that this
custom platform description is not found [2].

Providing such a custom platform description is what the upstream cmake
devs suggest [3], quoting:

    If a toolchain file specifies CMAKE_SYSTEM_NAME such that a custom
    `Platform/MySystem.cmake` file is loaded then the latter can set
    them [*] as needed for the target platform.

[*] offending settings causing RPATH issues during cross-compilation.

So we need to append our source tree to the module path, not replace it
blindly.

[0] https://buildroot.org/
[1] https://git.buildroot.org/buildroot/tree/support/misc/Buildroot.cmake
[2] http://autobuild.buildroot.net/results/69f/69fb2e3b549a069e2898506db918423e6742c589/build-end.log
[3] http://public.kitware.com/pipermail/cmake/2017-February/065063.html

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>

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