Update to 3.7.8
USE_GCC=yes causes the build to fail on systems where clang and libc++ are
used by default (see bug 202143, for example). This essentially means
comms/gnuradio currently only builds on 9.x. The problem is caused by the
fact that on 10.x and later boost is likely built using clang and libc++,
but USE_GCC=yes forces gnuradio to be built with GCC and libstdc++. Ports
r392951 mentioned USE_GCC=yes was added because "clang's builtins were
conflicting with gnuradio", which does not seem to happen in my tests at least.

PR: ports/203500
Submitted by: takefu@airport.fm