Alternative symlink-creating mode for file(INSTALL ...)

An new environment variable 'CMAKE_INSTALL_MODE' is introduced,
which can be used to ask CMake to create symbolic links
instead of copying files during a file(INSTALL ...).

The operation is at the file level only, directory trees are
still created using actual directories, not links.

Signed-off-by: Felix Lelchuk <felix.lelchuk@gmx.de>
38 files changed