For some reason the headers for agbcc are being installed in
tools/agbcc instead of tools/agbcc/include. This is a simple
(potentially temporary) workaround.
The compiler flag -s is not necessary and causes build failure on some
systems (macOS, clang compilers) due to it being deprecated.
Additionally, on those same systems, tools/aif2pcm/main.c failed to
compile due to an overly cautious compiler not liking some struct
declarations.
These changes afaik do not affect other builds.