Avoid having two of the same set of commands run in parallel
Avoid having two of the same set of commands run in parallel

A rule like this:

    _gen/x.c _gen/x.h: prerequisites
        $(AM_V_GEN)x-generator

doesn't consider x.c and x.h together. Instead, it expands to two rules,
one to generate x.c and one to generate x.h, which happen to run the
same commands.

This means that in the worst case, you can end up running x-generator
twice in parallel, and they'll race with each other and overwrite or
delete each other's output.

Based on commit 36c2a545c from telepathy-glib.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=64285
Signed-off-by: Simon McVittie <2b470b34502dbf3ba4bcc04ea7e64f4e0e104f33@collabora.co.uk>
Reviewed-by: Xavier Claessens <6f61b6f8ca6f7cbeae6bb087ddf85cf85dce43eb@collabora.co.uk>

freedesktop-unofficial-mirror/telepathy__telepathy-rakia,freedesktop-unofficial-mirror/telepathy__telepathy-rakia,freedesktop-unofficial-mirror/telepathy__telepathy-rakia