Handle parallel make properly

[robe]

I've seen this come up a number of times on IRC and once on mailing list:

​https://lists.osgeo.org/pipermail/postgis-devel/2015-October/025368.html

---

I did note that the resulting makefiles are incomplete, and I have to compile with 'make -j1' rather than the usual (for my build system) 'make -j12'.

---

Where people's make defaults to parallel build, and then PostGIS compile fails with a cryptic error I think about raster. I can't find the irc log entries where someone provided the error.

Is ther something we can do about this to either automatically disable this or allow it without error.

[strk]

Can be disabled: ​http://www.gnu.org/software/make/manual/make.html#Parallel

[strk]

(In [14284]) Properly encode raster/rt_pg dependencies

Fixes #3335 for me (in 2.3.0dev)

[strk]

NOTE: the fix makes parallel builds succeed here, and I could not find a trace of duplicated work (two threads doing the same thing twice) — but I might not have looked carefully enough

Tests welcome

[robe]

So if this works, we could backport to 2.1 right or you see an issue with that. We can configure travis to do parallel builds I think or at least I thought I saw mloskot do that for geos.

[strk]

I don't feel any need to backport it, but it should be safe to do. Having some bot build in parallel might help to find out how much it speeds up build times (assuming we can see info about times multiple builds in a single page)