various packages: fix inter-project macro dependencies

Solves this mysterious error on some operations such as "make update":

Makefile:273: *** Unknown archive format: /home/brand/projects/mxe/mxe/pkg/.  Stop.

This problem affected packages whose macros refer to other package
macros that were not yet defined due to the second package's .mk file
not being included before the first package's .mk file. Now we include
the file with the required macros explicitly on-time. The included .mk
file now has a multiple-inclusion guard.