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The contrib build system relies on pkg-config to detect system libraries, as done here:

ifeq ($(call need_pkg,'libnatpmp'),)
PKGS_FOUND += natpmp
endif

This never marks natpmp as found, because libnatpmp doesn't install any pkg-config file (.pc), thus cannot be found via pkg-config.

Possible solution: Rather than reinvent the wheel in this contrib build system, perhaps we could use Autoconf as the configuration driver of the contribs build system, which is quite capable in this regard. It could return a list of targets needed from the contribs build system to be built, and add them as dependencies.