compiler: do not use the outnames of fictive modules

Signed-off-by: Jean Privat <jean@pryen.org>
src: new module mixin to factorize the -m option

Signed-off-by: Jean Privat <jean@pryen.org>
mixin: introduce -D option to redefine functions from the command-line

Signed-off-by: Jean Privat <jean@pryen.org>
tests: test --define in engines

Signed-off-by: Jean Privat <jean@pryen.org>