Added -DHAVE_ALLOCA_H flag to libpd-osx target

Conflicts:
	libpd.xcodeproj/project.pbxproj

This is related to https://github.com/libpd/libpd/commit/e63874e444c2a8116c354a379204e6668a0ac835 and https://github.com/libpd/libpd/issues/57 The osx target needed the DHAVE_ALLOCA_H flag as well.