Fails to link with a custom install prefix.

[drmpeg]

The correct way to link with gr_filter is as follows:

1) In the top level CMakeLists,txt, add the line:

find_package(Gnuradio COMPONENTS fft filter blocks)

after the line:

find_package(Gnuradio "3.9.2" REQUIRED)

2) In lib/CMakeLists.txt, change gnuradio-filter to gnuradio::gnuradio-filter

[marcusmueller]

fix in #4