Log Message:
textproc/libxls: update to 1.5.1

Upstream changes:
 - Fix undefined behavior when formatting NaNs
 - Add missing files to distribution tarballs
 - Remove unused checks from configure
 - Add C++ test target and fix errors
 - Add a pkg-config file
 - Fix off-by-one in double byteswapping (big-endian systems)
 - Add note about Autoconf Archive

Package changes:
 - Add test target