CMake: installation of man pages

[rouault]

Slightly related to #5484, but not completely, as reported by @sebastic :
"There doesn't seem to be an equivalent for the install-man target, there is no CMakeLists.txt in man/man1 nor is that directory included in gdal.cmake. "

Important to note: the man/ directory is not included in the git repository, but is only a by product of mkgdaldist that runs "make man" in docs/ (which requires Sphinx). So CMake support for installing man pages should be conditioned to the presence of the man/ directory (only found in tarballs)