cf: Make local copy of ax_gcc_func_attribute.m4

Make a local copy of ax_gcc_func_attribute from autoconf-archive. This
is needed in order to fix a bug in the detection of the fallthrough
attribute.

Remove ax_gcc_func_attribute.m4 from src/external/autoconf-archive/m4.
Update LICENSE file to point to the local copy in src/cf.

Change-Id: I6c4244d2cd4edab4262c1820435c00419d85303b
Reviewed-on: https://gerrit.openafs.org/14272
Tested-by: BuildBot <[email protected]>
Reviewed-by: Andrew Deason <[email protected]>
Reviewed-by: Benjamin Kaduk <[email protected]>

Author: CheyenneWills

LICENSE

@@ -418,8 +418,8 @@ src/cf/lib-pathname.m4 are covered by the following license:
 
 ====================================================
 
-The files under src/external/autoconf-archive/m4/ are covered by the following
-license:
+The file src/cf/ax_gcc_func_attribute.m4 and the files under
+src/external/autoconf-archive/m4/ are covered by the following license:
 
   Copyright (c) 2008 Guido U. Draheim <[email protected]>
   Copyright (c) 2011 Maarten Bosmans <[email protected]>

src/external/autoconf-archive/m4/ax_gcc_func_attribute.m4 src/cf/ax_gcc_func_attribute.m4

@@ -1,5 +1,4 @@
 m4/ax_append_compile_flags.m4	m4/ax_append_compile_flags.m4
 m4/ax_append_flag.m4		m4/ax_append_flag.m4
 m4/ax_check_compile_flag.m4	m4/ax_check_compile_flag.m4
-m4/ax_gcc_func_attribute.m4	m4/ax_gcc_func_attribute.m4
 m4/ax_require_defined.m4	m4/ax_require_defined.m4