cmake: Refer to prefix variable in generated libqrencode.pc

[hebasto]

This change:

1. Makes the libqrencode.pc files generated by Autotools and CMake more aligned.
2. Allows the prefix variable to be redefined if the package is relocated.

For example:

$ cmake -B build
$ cmake --build build
$ cmake --install build --prefix /tmp/INSTALL
$ env PKG_CONFIG_PATH=/tmp/INSTALL/lib/pkgconfig pkg-config --cflags --libs libqrencode
-I/usr/local/include -L/usr/local/lib -lqrencode
$ mv /tmp/INSTALL /tmp/foo
$ env PKG_CONFIG_PATH=/tmp/foo/lib/pkgconfig pkg-config --define-variable=prefix=/tmp/foo --libs --cflags libqrencode 
-I/tmp/foo/include -L/tmp/foo/lib -lqrencode