CMake picked some homebrew libs in libzip building

Hi,

I've pulled new commits and download qgis-deps-0.2.1 in order to build the dependencies. I've found the following issue when running qgis_deps.bash, similar to https://github.com/qgis/QGIS-Mac-Packager/issues/38 . I manually unlink libnetttle from homebrew to check if I could fix it, but I got the same result.

Run build
Skipped build_bison
Skipped build_exiv2
Skipped build_freexl
Skipped build_geos
Skipped build_gsl
Skipped build_hdf5
Skipped build_jpeg
Skipped build_libffi
Skipped build_libtasn1
Skipped build_libxml2
Skipped build_netcdf
Skipped build_openssl
Skipped build_png
Skipped build_postgres
Skipped build_proj
Skipped build_qscintilla
Skipped build_qtkeychain
Skipped build_qwt
Skipped build_spatialindex
Skipped build_sqlite
Skipped build_webp
Skipped build_xerces
Skipped build_xz
Skipped build_zlib
Skipped build_zstd
Skipped build_flex
Skipped build_libtiff
Skipped build_libxslt
Call build_libzip
Entering in build environment
-- Configuring done
-- Generating done
-- Build files have been written to: /opt/QGIS/qgis-deps-0.2.1/build/libzip/build-x86_64
Found:
NETTLE_LIBRARY:FILEPATH=/usr/local/lib/libnettle.dylib
FIND_PACKAGE_MESSAGE_DETAILS_NETTLE:INTERNAL=[/usr/local/lib/libnettle.dylib][/usr/local/include][v()]
Leaving build environment
File CMakeCache.txt contains /usr/local/lib string <-- CMake picked some homebrew libs!

qgis / QGIS-Mac-Packager

CMake picked some homebrew libs in libzip building #39