repository cleanup

[umlaeute]

please consider:

• removing the file m_pd.h from the repository
  (shipping this header-file will get people into trouble who want to compile your external against a different version of Pd)
• removing any build-artifacts from the repository (plts.pd_darwin)
  (keeping these in the archive will create version-conflicts for everybody who builds plaits for Darwin; if you want to provide an easy download for people who don't know how to compile themselves, github allows to add release-artifacts to releases; you could also consider uploading the package to deken to make the lib very accessible)
• use PDLIBBUILDER_DIR to declare the path to the Makefile.pdlibbuilder, as documented. something like the following allows for easier integration on a variety of systems:

PDLIBDUILDER_DIR=.
include $(PDLIBBUILDER_DIR)/Makefile.pdlibbuilder