BUILD: Makefile: have "make clean" destroy .o/.a/.s in contrib subdirs as well

BUILD: Makefile: have "make clean" destroy .o/.a/.s in contrib subdirs as well

Now that we sometimes link some contrib subparts directly into the
haproxy binary, it's becoming a real problem that they're not cleaned
on make clean.  Some of the tools there are useful as .so or pure
binaries and we don't want to remove them, but anything intermediary
susceptible to be linked into haproxy should be clenaed. This is what
this patch does for 3 levels of subdirs into contrib/, without touching
the rest. It should be sufficient for the vast majority of use cases.