Adding rpkg build support for fedora or other rpm based distros

[theotheroracle]

in my fork, i've set up libdecsync to compile to a copr repo here and rpkg can be used to build or create other copr repos if necessary, making packaging for fedora or other rpm based distros quite easy .

i've tried to keep changes pretty minimal, the change mentioned in #3 was necessary for building Evolution-Decsync though, but that's not strictly necessary for this pr, and can be removed if needed .

the change to the makefile shouldn't affect it in any condition except the explicit definition of an alternate libdir, making packaging easier on fedora or other build systems which specify the libdir at buildtime .