Install man pages with CMake.
Install man pages with CMake.

Change-Id: I58a91ea00d45e1ce95250bd73613cf53e3af8401

PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp