Fix make race conditions reharding lang.h

The bug is due to a stupid make misfeature. The article [1] contains much more
information but in a nutshell, the following code:

a b: c
	bla

is equivalent to:

a: c
	bla
b: c
	bla

This is bad because in parallel runs (make -j typically), "bla" can be run
TWICE and even worse, twice in PARALLEL. Obviously the result will be
completely unexpected. This is a real bummer because on the other hand,
the following code:

%.c %.h: %:in
	bla

actually expresses the fact that bla produces two files. For some reasons,
pattern rules work differently from implicit rules.

This commit attempts to fix the problem with lang.h by rewriting (simplified):

lang.c lang.h: lang.in
	genlang

as

lang.h: lang.in
	genlang
lang.c: lang.h

This works (it correctly expresses the dependency chain and ensures genlang runs
once) but as one drawback: if one manually removes lang.c, then genlang will not
re-run since the second rule does nothing. This is minor drawback since no one
ever removes lang.c manually and "clean" removes lang.h which triggers a rebuild.

[1]: https://www.gnu.org/software/automake/manual/html_node/Multiple-Outputs.html

Change-Id: Ic0bf7c7c203dc599b00fde457946d2316c70474e

0 files changed, 0 insertions, 0 deletions