1) The ExternalProject_Add() command was called with
UPDATE_COMMAND ";" which was not noticable on most platforms
2) On AIX/GCC, the executable did not link because symbols from
imported libraries were assumed extern "C" (see commit 4fc47424)
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>