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