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Note that for the testing step (running the ctest command) you need to be connected to the internet (and set the relevant "http_proxy" environment variable if you must use a proxy) so data files can be downloaded for the tests. You can however download all the data files in one go (See the data tarball link at the top of the Releases page)
Also note that if you have several CPUs, you can invoke the "make" and "ctest" commands above in parallel (e.g. "make -j4"). This will speed up the whole build/test process significantly.
Legacy build system ("autotools")
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