Add --enable-shared option to iosbuild.sh to build dynamic framework

Also allows use of --enable-shared when configuring for Mac OS X,
producing a bare .dylib.

Enabling the shared framework bumps the iOS deployment target to 8.0,
the minimum required to support dynamic framework deployment in apps.

When not using --enable-shared, a static library for iOS 6.0+ will still
be built.

Minimum version settings have been moved into ios-version.sh so they
can be updated in a single place.

As with the static build, unless header search paths are manually
tweaked, users must add a VPX prefix on includes, such as:

  #include <VPX/vpx/vpx_decoder.h>

A module map for headers is not yet included as inttypes.h is not
modular; this means that VPX cannot be used directly in Swift code,
but can still be pulled in through an Objective-C wrapper.

BUG=https://bugs.chromium.org/p/webm/issues/detail?id=1092
Change-Id: I28fb06ce65e48ed167a88c14a7bfb2861989317e