Fix compiler warnings (#31)

.github/workflows/cmake-multi-platform.yml

CHANGELOG.md

CMakeLists.txt

cmake/config.h.in

cmake/libaec-config.cmake.in

include/libaec.h.in

src/CMakeLists.txt

src/Makefile.am

src/decode.c

src/encode.c

src/sz_compat.c

src/vector.h

tests/check_rsi_block_access.c

tests/check_seeking.c