build: always export TARGET_OS to child processes
build: always export TARGET_OS to child processes

TARGET_OS needs to be exported so it is always available in external
Makefiles.

Fixes TARGET_OS being uninitialized in external/Makefile when the root
Makefile is called without TARGET_OS specified.

gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl