enable parallel builds

[zbigniew]

Patch adds support for reliable parallel builds. At the least, it does not hurt normal builds.

[felix]

I think the rule that makes some .c files depend on setup-api.c might break building from a release tarball. Has this case been tested?

[zbigniew]

No, thanks for the tip. I will look into it.

[zbigniew]

It works as far as I can tell. Created a dist tarball and then built from it -- build process is attached.

Only thing to note is at -j2 or higher, it is not legal to do make install from a clean tarball, as the install target does not depend on any of the output files. However, it is legal to do make all install. At -j1, plain make install works fine.

I repeated this build process several times and Mario says it worked for him too.