You can specify --with-samba-source but the tests use a hard coded location that expects the test to be in examples/VFS or some location two levels deep in the Samba source tree.

The following patch would seem like a better way to do this:

diff --git a/examples/VFS/configure.in b/examples/VFS/configure.in
index 4e9d465..9e30197 100644
--- a/examples/VFS/configure.in
+++ b/examples/VFS/configure.in
@@ -337,11 +337,11 @@ AC_CACHE_CHECK([whether building shared libraries actually
    ac_cv_shlib_works=no
    # try building a trivial shared library
    if test "$PICSUFFIX" = "po"; then
-     $CC $CPPFLAGS $CFLAGS $PICFLAGS -c -o shlib.po ${srcdir-.}/../../tests/shl
+     $CC $CPPFLAGS $CFLAGS $PICFLAGS -c -o shlib.po ${SAMBA_SOURCE}/../tests/sh
        $CC $CPPFLAGS $CFLAGS `eval echo $LDSHFLAGS` -o "shlib.$SHLIBEXT" shlib.
        ac_cv_shlib_works=yes
    else
-     $CC $CPPFLAGS $CFLAGS $PICFLAGS -c -o shlib.$PICSUFFIX ${srcdir-.}/../../t
+     $CC $CPPFLAGS $CFLAGS $PICFLAGS -c -o shlib.$PICSUFFIX ${SAMBA_SOURCE}/../
        mv shlib.$PICSUFFIX shlib.po &&
        $CC $CPPFLAGS $CFLAGS `eval echo $LDSHFLAGS` -o "shlib.$SHLIBEXT" shlib.
        ac_cv_shlib_works=yes
@@ -350,6 +350,7 @@ AC_CACHE_CHECK([whether building shared libraries actually w
 ])
 if test $ac_cv_shlib_works = no; then
    BLDSHARED=false
+   AC_MSG_WARN([[Did you forget to specify --with-samba-source=/path/to/samba?]
 fi
 fi