Fix helper module auto-detection algorithm (#1194)

Fix helper module auto-detection algorithm (#1194)

Two issues here causing no helpers to be detected as buildable when
./configure parameters are missing entirely (eg "./configure ").
These were not detected by our build farm because missing helpers
is not an error, they are optional, and we lack tests validating
optional helpers.

1) Over-escaping of "$" variable prefix (M4 "$$" expands to "$")
   prevented the path string being expanded to correct value for
   the helper directory test. Causing it to always fail.

2) The $ac_top_srcdir variable internal to autoconf may not be set
   at the time AC_DEFUN macros are expanded. Making it unreliable.
   The $top_srcdir variable which should be used instead has the
   same problem with "top" not yet being determined. As such we
   use $srcdir instead and hope it expands correctly at ./configure
   run time.