libutf: fixes for warnings under mingw32-make
build: optional file extension in config.mk
libutf: only set cc if it is unset
project: hold onto dir path, not dirfd

opening a directory FD doesn't work on windows apparently.
build: config that works with mingw32-make
build: update gitignore for mdl.exe
project: store dir, not dir_str
doc: more readme
project: fix dir handling