gawk: update to 5.1.0.

Changes from 5.0.1 to 5.1.0
---------------------------

1. The major version of the API is bumped to 3, something that should
  have happened at the 5.0.0 release but didn't.

2. A number of memory leak issues have been fixed.

3. Infrastructure upgrades: Bison 3.5.4, Texinfo 6.7, Gettext 0.20.1,
  Automake 1.16.2.

4. The indexing in the manual has been thoroughly revised, in particular
  making use of the facilities in Texinfo 6.7.  That version (or newer)
  must be used to format the manual.

5. MSYS2 is now supported out-of-the-box by configure.

6. Several bugs have been fixed.  See the ChangeLog for details.

(wiz)