\li \c PROTO_FILES is the list of \c .proto files that will be used in the
generation procedure.
\li \c PROTO_INCLUDES is the list of directories that will be searched for
dependencies.
\li \c OUTPUT_DIRECTORY is the directory where the generated files will be
put. By default, the current directory (while evaluating the function) is
used.
\li \c OUTPUT_HEADERS can be used to specify a variable that will
hold the list of headers created by the function. This list can be useful for
custom project install rules.
\li \c OUTPUT_TARGETS can be used to specify a variable that will
hold the list of targets created by the function. This list can be useful for
custom project install rules.
