Log Message:
graphics/autopano-sift-C: Install manpages into ${PKGMANDIR}.

This package uses CMake to configure the software.  Include the
GNUInstallDirs CMake module to define CMAKE_INSTALL_<dir> macros,
and use CMAKE_INSTALL_MANDIR as the location for the installed
manpages.  The correct environment variables are passed so that
${CMAKE_INSTALL_MANDIR} points into ${PKGMANDIR}.