allow build folder as $BUILD_DIR

[PierreZ]

This PR is adding the build folder in the .gitignore, allowing IDE to use the standard $SOURCE_DIR/build, as discussed in #3098.

I didn't find any reference to a builds folder, so I changed it to build.

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[ammolitor]

Looks good!

[sfc-gh-mpilman]

The .gitignore change is probably a good one, but the documentation change is not advocating for what we consider best practice.

[foundationdb-ci]

AWS CodeBuild CI Report

• CodeBuild project: foundationdb-pull-request-build-macos
• Commit ID: 072ed78
• Result: FAILED
• Build Logs (available for 7 days)

[foundationdb-ci]

AWS CodeBuild CI Report

• CodeBuild project: foundationdb-pull-request-build
• Commit ID: 072ed78
• Result: SUCCEEDED
• Build Logs (available for 7 days)