build: make check for BSD fstatfs() a bit more strict

Linux also has fstatfs(), and the test relied on certain system include
files being available on BSD, but not on Linux. It would break if Linux
added the missing includes for some reason.

Make it a bit stricter, and check for the struct statfs field the code
needs.

wscript

@@ -200,7 +200,7 @@ iconv support use --disable-iconv.",
         'name': 'bsd-fstatfs',
         'desc': "BSD's fstatfs()",
         'func': check_statement(['sys/param.h', 'sys/mount.h'],
-                                'struct statfs fs; fstatfs(0, &fs)')
+                                'struct statfs fs; fstatfs(0, &fs); fs.f_fstypename')
     }, {
         'name': 'linux-fstatfs',
         'desc': "Linux's fstatfs()",