build: Run lopsubgen with absolute path.

build: Run lopsubgen with absolute path.

The configure script detects the location of the lopsubgen executable,
but we never use this information and rely on the shell to find the
executable in $PATH when the recipes in m4/lls/makefile are run.

This should not matter much, but it is inconsistent to how we deal
with m4. Moreover, using the LOPSUBGEN autoconf variable in the
recipes allows the user to override the path at build time.