Support creation of depfiles for use with make and ninja based builds

[markus-mayer-bl]

When compiling sass files with make or ninja based buildsystems, it would be nice if dart-sass would be invoked only if a scss file (or one if its dependencies) has changed.

This can be achieved by so called "depfiles". These files are written by the compiler (dart-sass in this case) during compilation and contain the dependencies of a source file.

I'm aware that dart-sass has the '--update' option, but it still takes some time to parse all the imports. If depfiles would be written by dart-sass, the buildsystem wouldn't even need to call dart-sass at the first place (if nothing has changed).

I'm willing to contribute this to dart-sass, but only if such a feature would be accepted at all.

[jathak]

Is there some sort of specification for depfiles that could allow them to (in principle) be used by a variety of build systems? If this is too specific to make/ninja, we're unlikely to accept it, but if it has utility beyond any one build system, it is something that we could merge if you were to implement it.

[markus-mayer-bl]

The format of the depfile is not formally specified, but there seems to be an agreed on format. It is a subset of the syntax of a Makefile. This is also the format that gcc generates when '-MD' is passed.

I did a quick google search and found the following buildsystems to support depfiles: make, ninja, meson (to some extend)