build: Error out on implicit function declarations.

Replacing lopsub.h by an empty file builds a buggy lopsubgen-stage1
executable which segfaults due to a format string mismatch caused
by the implicit declaration of lls_version(). In this package all
functions are supposed to be declared, so let's fail the build if
any undeclared functions are found.

Tested-by: Alex Gietz <oss@mcdinner.de>