This patch adds support for building shared libraries when configured
with the --enable-shared switch.

Building DLLs would require more invasive changes to the sample
utilities than I want to make in this patch, since on Windows you can't
use the address of an imported symbol in a static initializer. The best
way to work around this is proably to build the codec interface mapping
table with an init() function, but dll support is of questionable value
anyway, since most windows users will probably use a media framework
lib like webmdshow, which links this library in staticly.

Change-Id: Iafb48900549b0c6b67f4a05d3b790b2643d026f4

Change-Id: Ieebea089095d9073b3a94932791099f614ce120c

Support --prefix, --libdir as a conventional way of specifying the default
installation directories. libdir is required to be a subdirectory of prefix
at this time.

Change-Id: If45d9e3129efcde83c05b7766accc9017988e715

This renames the vpx_codec/ directory to vpx/, to allow applications
to more consistently reference these includes with the vpx/ prefix.
This allows the includes to be installed in /usr/local/include/vpx
rather than polluting the system includes directory with an
excessive number of includes.

Change-Id: I7b0652a20543d93f38f421c60b0bbccde4d61b4f

This doesn't play well with autotools, and the preprocessor magic is
confusing and unhelpful in the vp8-only context.

Change-Id: I2fcb57e6eb7876ecb58509da608dc21f26077ff1