GNU Astronomy Utilities manual

3.2 Optional requirements

Most of the programs in Gnuastro make use of the libraries in Requirements, therefore if they are not available, the configure script will complain and compiling the Gnuastro is not possible. The libraries listed in this section are only used for very specific applications, therefore if you don’t want these operations, they do not need to be present.

If the ./configure script can’t find these requirements, it will warn you that they are not present and notify you of the operation(s) you can’t do due to not having them. If the output you request from a program requires a missing library, that program is going to warn you and abort. In the case of executables like GPL GhostScript, if you install them at a later time, the program will run. This is because if required libraries are not present at build time, the executables cannot be built, but an executable is called by the built program at run time so if it becomes available, it will be used. If you do install an optional library later, you will have to rebuild Gnuastro and reinstall it for it to take effect.

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GNU Astronomy Utilities manual, May 2016.