libacl: work with older versions, too

this was recently set to a relatively high minimum version when
locating it via pkgconfig was added. this broke the binary builds
on buster and bullseye.

i don't think borg requires a specific libacl version as long as
the api is compatible, so i now set this to 2.2.47 (from 2008).

setup.py

@@ -156,7 +156,7 @@ if not on_rtd:
     if sys.platform == "linux":
         linux_ext_kwargs = members_appended(
             dict(sources=[platform_linux_source]),
-            lib_ext_kwargs(pc, "BORG_LIBACL_PREFIX", "acl", "libacl", ">=2.3.1"),
+            lib_ext_kwargs(pc, "BORG_LIBACL_PREFIX", "acl", "libacl", ">= 2.2.47"),
             dict(extra_compile_args=cflags),
         )
     else: