Rebuild libextrl-mkexports every time, to make it pick up changes in system.mk
Rebuild libextrl-mkexports every time, to make it pick up changes in system.mk

A 'make clean; make' will rebuild everything already, but not
libextrl-mkexports since it doesn't depend on a cleaned artifact.

jsbackus/notion-libextl,jsbackus/notion-libextl