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/surfaces/frontier/wscript

@@ -35,8 +35,6 @@ def build(bld):
     '''
     obj.export_includes = ['./generic_midi']
     obj.defines     = [ 'PACKAGE="ardour_frontier"' ]
-    if bld.is_defined ('INTERNAL_SHARED_LIBS'):
-        obj.defines += [ 'LIBARDOUR_DLL=1', 'LIBCONTROLCP_DLL=1', 'LIBPBD_DLL=1' ]
     obj.includes     = ['.', './generic_midi']
     obj.name         = 'libgeneric_midi'
     obj.target       = 'generic_midi'