build: tidy up header paths, use srcdir where possible

[vszakats]

To improve readability.

Also add more comments on why each is necessary.

[vszakats]

This message in lib/Makefile.am scared me off first:

# Specify our include paths here, and do it relative to $(top_srcdir) and
# $(top_builddir), to ensure that these paths which belong to the library
# being currently built and tested are searched before the library which
# might possibly already be installed in the system.

via 1e7125a

But testing shows that $(srcdir) and $(top_srcdir)/lib end up being the same in the verbose build log (both relative).
Same is true for CMake for ${CMAKE_CURRENT_SOURCE_DIR} vs. ${PROJECT_SOURCE_DIR}/tests/libtest (both absolute).