Go: do not clobber GOFLAGS in AC_PROG_GO (sr #111383)

The AC_PROG_GO macro should honor the GOFLAGS set by the user at
configure time.

Fixes <https://savannah.gnu.org/support/?111382>.

* lib/autoconf/go.m4: Do not clobber GOFLAGS in AC_PROG_GO.
  Simplify sequence of AC_CHECK_TOOL/AC_CHECK_PROGS operations;
  this macro does not have the backward compatibility constraints
  that AC_PROG_CC does.