c_CholmodFactor and CholmodFactor depend on version of CHOLMOD

In version 2.1.0 of the CHOLMOD library of C functions a new field was
added to the cholmod_factor struct.  For compatibility the definitions
of the c_CholmodFactor and CholmodFactor types now depend on the
version of the CHOLMOD headers through the value of a trivial C
function jl_cholmod_version defined in deps/Suitesparse_wrapper.c

This is not ideal.  It would be better for the shared object file
libcholmod.so to contain such a function so that the version of the
code in the shared object can be checked and not just the version of
the headers against which deps/Suitesparse_wrapper.c was compiled.

The suitesparse tests are reinstated.  Also mistakes related to the x
and px fields in the external constructor for the CholmodFactor object
from Ptr{c_CholmodFactor} were corrected and the code for the logdet
methods was changed to use the diag extractor.  This allows the logdet
of supernodal factorizations to be evaluated.