Made makefile use default pdf-viewer on Linux and macOS
Made makefile use default pdf-viewer on Linux and macOS. Currently the makefile only works if Evince is installed.
No Windows support in this makefile since "uname -s" won't execute unless running in some POSIX compliant shell. If running a POSIX shell with support for launching GUI applications on Windows (not Ubuntu bash but for example Cygwin) the makefile should work using the command start
as OPEN-CMD. This is not tested.