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/pbd/wscript

@@ -114,9 +114,7 @@ def build(bld):
     if bld.is_defined ('INTERNAL_SHARED_LIBS'):
         print('BUILD SHARED LIB')
         obj              = bld.shlib(features = 'cxx cxxshlib', source=libpbd_sources)
-        obj.defines = [ 'LIBPBD_DLL=1',
-                        'LIBPBD_DLL_EXPORTS=1' 
-                        ]
+        obj.defines = [ 'LIBPBD_DLL_EXPORTS=1' ]
         obj.cxxflags = [ '-fvisibility=hidden' ]
         obj.cflags = [ '-fvisibility=hidden' ]
     else: