check for all necessary software at once, enhanced README.md

[freckhard]

Hey there, thanks for your icon repository for libreoffice, finally my poor eyes can see pretty icons with dark mode! I was building the icon set myself, but had to rerun the build.sh three times individually because I lacked the dependencies (and did not check the build.sh). So my addition checks for all of those (inlcuding inkscape, which was somehow necessary, but not installed on my mint desktop) and aborts if necessary after all have been checked. I also added some minor additional details to the README.md, listing the prerequisites.

[rizmut]

Thanks for addition. For light only build script I would rather to see some option like build.sh --l. Would be great if you know how to add that kind of option.