drop the lame ADD_*_CPPFLAGS and FORCE_*_LDFLAGS for the less weird
PKG_*_CPPFLAGS and PKG_*_LDFLAGS.
Also, in the process, fix an annoying bug where setting PKG_FOO_CPPFLAGS
(where FOO can be fulfilled by BAR and BAZ) wouldn't be propagated to
PKG_BAR_CPPFLAGS. This way, people don't have to know which of the
alternatives to pass in CPPFLAGS/LDFLAGS for.