consolidate zero-length arrays (aka struct hack)

consolidate zero-length arrays (aka struct hack)

HLT uses a lot of structs with zero-length arrays, using #ifdef for
platforms not supporting those (setting array_t array[1] instead of
array[0]). These ifdef were inconsistent. Sometimes also array[] instead
of array[0] was used. Moreover, for some structs clang was excluded
(defined(__clang__)), which lead to inconsistencies when calculating sizes
with sizeof() and ultimately runtime errors (output buffer too small in
TPC cluster transformation component). This patch tries to make all
zero-length array structs consistent and removes exceptions for clang.

Whether or not the __SUNPRO et al. defines are still needed is unclear,
maybe they can be removed in the future.