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.

Rather than compiling in the fixed version number from the configure
file, it's more helpful for testing and debugging to directly embed
the repository's revision. We use 'git describe --tags' for this, since
for tagged revisions it just returns the tag name, and in other cases
is more readable than a raw commit id.

If git isn't present or can't find a repository to query, we fall back
to the static version number in the configure script.

Ralph Giles authored 13 years ago and Jean-Marc Valin committed 13 years ago

Automake version 1.11 and above provide a build mode where
each build command is replaced by a single-line annotation.
Since autotools build lines for libraries typically each fill
an entire terminal screen, this mode makes it easier to keep
track of build progress, and warnings stand out more clearly.

For debugging the build process, the old behaviour can be
recovered by running 'make V=1' at bulid time, or by passing
--disable-silent-rules to configure.

Since Automake 1.11 is still very recent, we only invoke
the AM_SILENT_RULES macro if it is defined, allowing fallback
to the old behaviour under previous versions of automake.

With minor fixes