axel: use the new gettext logic

axel: use the new gettext logic

This commit switches to use the new gettext logic, which involves:

 - using TARGET_NLS_DEPENDENCIES instead of hand-encoded dependencies
   on gettext/host-gettext

 - using TARGET_NLS_LIBS to force linking against libintl

 - dropping BR2_PACKAGE_GETTEXT selection

Note that -lpthread is no longer needed for -lintl, but axel uses
pthread directly anyway, so we have to keep -lpthread as part of
LDFLAGS.

Finally, we now use BR2_SYSTEM_ENABLE_NLS to decide whether NLS
support should be enabled or not.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>