Recently, FindBLAS has been extended with additional library search
path based on the environment variable MKLROOT. However, the choice
of the Intel MKL architecture (IA-32 vs Intel64) was based on
unrelated (and possibly undefined) size of integer.

This commit changes the selection of the Intel MKL architecture to
instead consider the variable BLA_VENDOR, if available.

So, if the environment variable MKLROOT is defined and
BLA_VENDOR=Intel10_32, then $ENV{MKLROOT}/lib/ia32_<OS> will be added
to the search path (OS = lin, win, or mac).

Similarly, if MKLROOT is defined and BLA_VENDOR=Intel10_64lp or
BLA_VENDOR=Intel10_64ilp, then the path $ENV{MKLROOT}/intel64_<OS>
will be used.

If either MKLROOT or BLA_VENDOR is undefined, no additional search
path on top of LD_LIBRARY_PATH / DYLD_LIBRARY_PATH / LIB is be added.