h5check

Improve configure process.

Problem:
The package needs h5cc to compile the test generation program to generate
test files. Configure (config/commence.am) used to hardcode the h5cc tool
as in $bindir/h5cc which was rarely true.

Solution:
I changed it to just use "h5cc" which uses the bigger $PATH scope.
One can still use the old way of "make H5CC=$mypath/h5cc".

Tested:
Jam, smirom, linew and even duty and liberty.
(I had updated the README of the change to using default
h5cc but did not add FreeBSD to the supported platform list
because there is no ongoing funding for FreeBSD platforms
support.)