vchoi_fork

[svn-r24368] Issue 8147 - clang's -fcatch-undefined-behavior exposed undefined behavior in the alignment detection algorithm for integers in H5detect.c.  Quincey decided to solve it in two steps.  First, catch illegal instruction signal in the original algorithm.  Second, use POSIX __alignof function to detect alignment if it's available.  This checkin is the first step.

Tested on wren, jam, ostrich, platypas, and koala.