Remove unnecessary if statement and fix libcaesium error

[4G3NT]

caesium-image-compressor: error while loading shared libraries: libcaesium.so: cannot open shared object file: No such file or directory

Install target doesn't copy libcaesium.so to the install prefix and symlinks it to /usr/lib/libcaesium.so, thus causing Caesium Image Compressor to fail to locate it.

[4G3NT]

If you have a more efficient way of doing this, please make a suggestion and I will apply it.

[Lymphatus]

I think it may break if you already have libcaesium in you lib path. If you look at #199 the goal here was to be able to build if you already had the library on your own. So maybe it's better check if already it's there before symlinking.

[4G3NT]

I tested it by having the Caesium Image Compressor already installed and with a dead symlink /usr/lib/libcaesium.so -> /usr/local/lib, and it worked fine. Can you test it yourself and see if that's the case?

[4G3NT]

before and after installation

[Lymphatus]

Seems ok to me, but I would just wrap the part where you install libcaesium in a IF (NOT LIBCAESIUM).
If someone has already a libcaesium installation I'd rather not mess with it and leave as it is.