If Halibut is not available to build the docs, but on the other hand
pre-built man pages already exist (e.g. because you unpacked a source
zip file with them already provided), then docs/CMakeLists.txt creates
a set of build rules that copy the pre-built man pages from the source
directory to the build directory.
However, if the source and build directories are the _same_, this
creates a set of cyclic dependencies, i.e. files which depend directly
on themselves. Some build tools (in particular 'ninja') will report
this as an error.
In that situation, the simple fix is to leave off the build rules
completely: if the man pages are already where the build will want
them to end up, there need not be any build rule to do anything about
them.
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