Set cmake minimum version to that which is really required.

The use of target_sources in combination with generator expressions
is not supported before v3.5. Set min required version to v3.5.

BUG=https://bugs.chromium.org/p/aomedia/issues/detail?id=76

Change-Id: Ifa1ea25e17ac58d631436c31ab9d4d9231e689f2
4 files changed