Fix dependencies, Makefile cleanups

Detailed maintainer log:

• Explicitly rely on required perl for the build
• math/R cannot be built in poudriere when math/atlas is chosen for the BLAS implementation. It's not the default, so handle gracefully with an IGNORE (could also have set MANUAL_PACKAGE_BUILD with ATLAS_VARS= MANUAL_PACKAGE_BUILD=...)
• fix a type: DOC_BUILD_DEPEND -> DOCS_BUILD_DEPEND

PR: 210983
Submitted by: jrm@ftfl.ca (maintainer)