[GRASS5] Non-portable code in 5.1 portability test

[cheg01 at attbi.com]

The file /lib/vector/diglib/test.c fails on Solaris. Under Solaris, you
cannot perform a write immediately after a on the same file without
performing fflush(fp) in between or closing and reopening the file.

Here is the diff output:

66a67
>             fflush(fp);
79a81
>             fflush(fp);
93a96
>             fflush(fp);
107a111
>             fflush(fp);
121a126
>             fflush(fp);
134a140
>             fflush(fp);