Set CMake 3.13 UseSWIG policy

[nunojpg]

CMake Warning (dev) at /usr/share/cmake-3.15/Modules/UseSWIG.cmake:607 (message):
Policy CMP0078 is not set: UseSWIG generates standard target names. Run
"cmake --help-policy CMP0078" for policy details. Use the cmake_policy
command to set the policy and suppress this warning.

Call Stack (most recent call first):
dependencies/SoapySDR/python/CMakeLists.txt:155 (SWIG_ADD_LIBRARY)
This warning is for project developers. Use -Wno-dev to suppress it.

CMake Warning (dev) at /usr/share/cmake-3.15/Modules/UseSWIG.cmake:460 (message):
Policy CMP0086 is not set: UseSWIG honors SWIG_MODULE_NAME via -module
flag. Run "cmake --help-policy CMP0086" for policy details. Use the
cmake_policy command to set the policy and suppress this warning.

Call Stack (most recent call first):
/usr/share/cmake-3.15/Modules/UseSWIG.cmake:702 (SWIG_ADD_SOURCE_TO_MODULE)
dependencies/SoapySDR/python/CMakeLists.txt:155 (SWIG_ADD_LIBRARY)
This warning is for project developers. Use -Wno-dev to suppress it.

The respective policies:
https://cmake.org/cmake/help/git-master/policy/CMP0078.html
https://cmake.org/cmake/help/git-master/policy/CMP0086.html

Unfortunately I can't provide a patch as I don't use Python and cannot test it.

[guruofquality]

Swig cmake stuff has changed more than once. Thanks for reporting. It doesnt look like an issue now -- just a warning, but I will try to find a way to make cmake future proof when the policy setting changes.