audio/liquidsfz: fix build on GCC architectures

GCC defines those, causing error, move those to be set only with Clang::
<command-line>: error: duplicate 'unsigned'
<command-line>: error: declaration does not declare anything [-fpermissive]
<command-line>: error: duplicate 'unsigned'
<command-line>: error: declaration does not declare anything [-fpermissive]