Fix incorrect type of index variable
Remove unused variables
Make BMP processing functions cross-platform
autotools: Add --enable-debug
Add pkgconfig file
autotools: Add all headers to SOURCES
autotools: Compile Unresize too
autotools: Compile TestApp too
Fix cross-compilation with mingw-w64
Presumably fix avx2 detection with gcc/clang
autotools: Don't use -export-symbols-regex

TestApp uses internal symbols.