Comment 4 for bug 1718668

gcc -DHAVE_CONFIG_H -DEXEEXT=\"\" -I. -I../../../../gnulib/tests -I../.. -DIN_LIBVIRT_GNULIB_TESTS=1 -I. -I../../../../gnulib/tests -I../.. -I../../../../gnulib/tests/../.. -I../../gnulib/lib -I../../../../gnulib/tests/../../gnulib/lib -g -O2 -c -o test-getline.o ../../../../gnulib/tests/test-getline.c
gcc -DHAVE_CONFIG_H -DEXEEXT=\"\" -I. -I../../../../gnulib/tests -I../.. -DIN_LIBVIRT_GNULIB_TESTS=1 -I. -I../../../../gnulib/tests -I../.. -I../../../../gnulib/tests/../.. -I../../gnulib/lib -I../../../../gnulib/tests/../../gnulib/lib -g -O2 -c -o test-getopt-posix.o ../../../../gnulib/tests/test-getopt-posix.c

/bin/bash ../../libtool --tag=CC --preserve-dup-deps --mode=link gcc -g -O2 -o test-getopt-posix test-getopt-posix.o libtests.a ../../gnulib/lib/libgnu.la libtests.a -ldl

With -E it generates:
# 86 "../../../../gnulib/tests/test-getopt.h"
      posixly = !!getenv ("POSIXLY_CORRECT");
  posixly =
# 92 "../../../../gnulib/tests/test-getopt.h" 3 4
           1
# 92 "../../../../gnulib/tests/test-getopt.h"

So it is at build time.
A short check shows that it is __GETOPT_PREFIX that is defined and bringing the hard assign in.
Also disabling the assignment makes the test work.

Need to find where the __GETOPT_PREFIX now comes from.