Do Pull-Request right and build the result of the merge of source and target.
Actually the builds blocking a PR does not give any sense of security. Because the build is only run for the source. The real safety is to run the builds against a merge of source and target. I dont know why this required feature is missing in bitbucket/bamboo - So here are the requirements:

• Run Builds of PRs aigainst the merge-result of source and target
• Let us define which Build-Plans must have green builds per target-branch.
• Let us choose when to invalidate a build (changes in source, target or in both)
• Let us choose to auto trigger a new build on build-invalidation

This is a must have feature for "real" source-control/build system.