Tell automake to put object files in subdirectories.

Previously the autotools build litered whatever directory
'make' was invoked from with intermediate files. Adding
'subdir-objects' to AUTOMAKE_OPTIONS tells it to put them
in subdirectories according to the relative paths to the
source files, .i.e. silk/schur.o for $(srcdir)/silk/schur.c

The change to autogen.sh is necessary to handle the
(apparently new) case where an AUTOMAKE_OPTIONS line is
given without a minimum version number.