fixed GVPS paths and updated README to not reference missing submodules

[nils-werner]

After your feedback on #7, this PR fixes the include paths to libgvps, and replaces any mention of submodules with an instruction on how to git clone and compile the two libraries.

[Sleepwalking]

Hello, thanks for the continued efforts. There in fact is no need to change the makefile or source code if you run make install in libgvps and use the same PREFIX for both libraries.

[nils-werner]

I don't want to run make install, but instead include them in a different project. One thing that is stopping me, is the path in

#include <libgvps/gvps.h>

Shouldn't the libgvps/ prefix be configurable using a parameter/variable?

Together with Makefile it produces an includepath /usr/include/libgvps of which I can only change the /usr/include part (/usr via $GVPS_PREFIX, /include only by modifying the Makefile). If libgvps lives somewhere else, I need to modify the C code first.

Also I don't see the point in installing .a libraries... .so of course, but .a libs don't make much sense I think.