reverse the visibility.h files assumptions that we don't build shared libs by default

This avoids having to define define LIBFOO_DLL=1 all over the place. If we ever go with static libs we will
need to define LIBFOO_STATIC=1 but hopefully in some central location like the top level wscript.

Oh, and I also dropped support for gcc older than version 4.x because ardour will already not build
on such an old version.

libs/panners/vbap/wscript

@@ -23,8 +23,6 @@ def build(bld):
     obj.source = [ 'vbap_speakers.cc', 'vbap.cc'  ]
     obj.export_includes = ['.']
     obj.defines     = [ 'PACKAGE="libardour_panvbap"' ]
-    if bld.is_defined ('INTERNAL_SHARED_LIBS'):
-        obj.defines += [ 'LIBARDOUR_DLL=1', 'LIBCONTROLCP_DLL_=1', 'LIBPBD_DLL=1', 'LIBEVORAL_DLL=1' ]
     obj.includes     = ['.']
     obj.name         = 'libardour_panvbap'
     obj.target       = 'panvbap'