Update --rebuild-if-* flags to rebuild when build dependencies are changed.

Update --rebuild-if-* flags to rebuild when build dependencies are changed.

Right now, the --rebuild-if-* flags only rebuild packages that are used at
both run-time and build-time. This doesn't help for packages that are used
only at build-time (for example, static libaries).

Rebuilding packages whenever a build-time dependency is changed is easier to
understand and explain, and it handles all cases correctly.

BUG=chromium-os:15517
TEST=Run emerge test suite.

Change-Id: Iae8dab24e8acb6625bc1a0ce41862e90b232eb84