build: don't add subdirectories to include search path but require qualified include
Keep the include paths clean and separate. We use directories to group source files together. That makes sense (I guess), but then we should use this grouping also when including files. Thus require to #include files with their path relative to "src/". Also, we build various artifacts from the "src/" tree. Instead of having individual CFLAGS for each artifact in Makefile.am, the CFLAGS should be unified. Previously, the CFLAGS for each artifact differ and are inconsistent in which paths they add to the search path. Fix the inconsistency by just don't add the paths at all.
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