cmake whining for CMP0077 for vendored libxmp and mpg123

[sezero]

Cmake is whining about CMP0077 (see the latest CI logs).

For vendored libxmp:

CMake Warning (dev) at external/libxmp/CMakeLists.txt:25 (option):
  Policy CMP0077 is not set: option() honors normal variables.  Run "cmake
  --help-policy CMP0077" for policy details.  Use the cmake_policy command to
  set the policy and suppress this warning.

  For compatibility with older versions of CMake, option is clearing the
  normal variable 'BUILD_STATIC'.
This warning is for project developers.  Use -Wno-dev to suppress it.

CMake Warning (dev) at external/libxmp/CMakeLists.txt:26 (option):
  Policy CMP0077 is not set: option() honors normal variables.  Run "cmake
  --help-policy CMP0077" for policy details.  Use the cmake_policy command to
  set the policy and suppress this warning.

  For compatibility with older versions of CMake, option is clearing the
  normal variable 'BUILD_SHARED'.
This warning is for project developers.  Use -Wno-dev to suppress it.

For vendored mpg123:

CMake Warning (dev) at external/mpg123/ports/cmake/src/libmpg123/CMakeLists.txt:4 (option):
  Policy CMP0077 is not set: option() honors normal variables.  Run "cmake
  --help-policy CMP0077" for policy details.  Use the cmake_policy command to
  set the policy and suppress this warning.

  For compatibility with older versions of CMake, option is clearing the
  normal variable 'NO_LFS_ALIAS'.

(Don't know whether SDL_image is affected.)

[madebr]

This is taken care of in b1f3661d

[sezero]

Same issue is in SDL2 branch too

[madebr]

24216e41