cygwin: build error with strerror_r since 0.11

libssh/src/libssh-0.11.0/src/misc.c: In function ‘ssh_strerror’:
libssh/src/libssh-0.11.0/src/misc.c:1984:8: error: assignment to ‘int’ from ‘char *’ makes integer from pointer without a cast [-Werror=int-conversion]
 1984 |     rv = strerror_r(err_num, buf, buflen);
      |        ^

The cmake build defines _GNU_SOURCE for cygwin, which makes strerror_r() return a char*