Do not hardcode /usr prefix

[probonopd]

https://github.com/JannikHv/gydl/blob/35744b8bd90c5b09f9c2f0431319e6a143fe696f/Makefile#L18-L28

Please do not hardcode the /usr prefix so that one can use something like

$ ./configure --prefix=/usr
$ make -j$(nproc)
$ make install DESTDIR=$(readlink -f appdir)

$ find appdir/
(...)
appdir/usr/bin
(...)