package/musepack: fix static link

- disable shared object build when BR2_PREFER_STATIC_LIB is set
- patch the CMake code for handling static/shared object build using
  standard CMake flags, instead of the custom ones.

Fixes:
  http://autobuild.buildroot.org/results/e5e/e5eef14d5ed76db824c47a7e679e68397a023526/



Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>