XXX_LIBRARIES The libraries to link against to use XXX. These should include full paths. This should not be a cache entry.
XXX_DEFINITIONS Definitions to use when compiling code that uses XXX. This really shouldn't include options such as (-DHAS_JPEG)that a client source-code file uses to decide whether to #include <jpeg.h>
XXX_EXECUTABLE Where to find the XXX tool.
XXX_YYY_EXECUTABLE Where to find the YYY tool that comes with XXX.
XXX_RUNTIME_LIBRARY_DIRS Optionally, the runtime library search path for use when running an executable linked to shared libraries.
The list should be used by user code to create the PATH on windows or LD_LIBRARY_PATH on unix.
This should not be a cache entry.
XXX_VERSION_STRING A human-readable string containing the version of the package found, if any.
XXX_VERSION_MAJOR The major version of the package found, if any.
XXX_VERSION_MINOR The minor version of the package found, if any.
XXX_VERSION_PATCH The patch version of the package found, if any.
You do not have to provide all of the above variables. You should provide XXX_FOUND under most circumstances. If XXX is a library, then XXX_LIBRARIES, should also be defined, and XXX_INCLUDE_DIRS should usually be defined (I guess libm.a might be an exception)
The following names should not usually be used in CMakeLists.txt files, but they may be usefully modified in users' CMake Caches to control stuff.
For tidiness's sake, try to keep as many options as possible out of the cache, leaving at least one option which can be used to disable use of the module, or locate a not-found library (e.g. XXX_ROOT_DIR). For the same reason, mark most cache options as advanced.
If you need other commands to do special things then it should still begin with XXX_. This gives a sort of namespace effect and keeps things tidy for the user. You should put comments describing all the exported settings, plus descriptions of any the users can use to control stuff.
You really should also provide backwards compatibility any old settings that were actually in use. Make sure you comment them as deprecated, so that no-one starts using them.
To correctly document a module, create a comment block at the top with # comments. There are three types of comments that can be in the block:
1. The brief description of the module, this is done by:
# - a small description
2. A paragraph of text. This is done with all text that has a single
space between the # and the text. To create a new paragraph, just
put a # with no text on the line.
3. A verbatim line. This is done with two spaces between the # and the text.