It turns out it's actually pdpmake-build-time-configurable. If you build with -DENABLE_FEATURE_MAKE_EXTENSIONS -DENABLE_FEATURE_MAKE_POSIX_202X, extensions are enabled by default, and .POSIX only flags non-POSIX-202x extensions. I don't know what, if anything, this says about the main point of pdpmake, but I'm glad with this mode.