Description of problem:

pkg-config --libs --static should take into account Requires.static
recursively. For example if one have, in the .pc file 

Libs: -L${libdir} -ltests
Requires.private: x11

One get:
$ pkg-config --static --libs tests
-ltests -lX11 -lpthread

While, there is
$ pkg-config --static --libs x11
-lX11 -lpthread -lXau -lXdmcp

One should have
$ pkg-config --static --libs tests
-ltests -lX11 -lpthread -lXau -lXdmcp

in the x11.pc there is
Requires.private: xau xdmcp
Libs.private: -lpthread

so -lpthread is taken into account, while Requires.private are
not taken into account.

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