Low: Build: Rename LINK_SOURCES to LINK_FILES.

_SOURCES means something to autoconf and results in the following error
messages:

lib/common/tests/schemas/Makefile.am:54: warning: variable 'LINK_SOURCES' is defined but no program or
lib/common/tests/schemas/Makefile.am:54: library has 'LINK' as canonical name (possible typo)