configure: clean up Altivec detection

There used to be one test for Altivec intrinsics support and a
separate test to determine which of two possible syntaxes to use
for vector literals.  Since 2008, we only support the more common
of these so the split test no longer makes sense.

This combines the tests into one and also changes the hard error on
failure to a warning.  The test can reasonably fail if no --cpu flag
is provided (or is provided with an unknown CPU) and the compiler
default target does not support Altivec.  Aborting in this case is
probably over-reacting.

Signed-off-by: Mans Rullgard <mans@mansr.com>