Prepending '+' to the recipe line when linking with GCC's -flto=jobserver

[R. Diez]

Hi all:

I am developing firmware similar to this one:

https://github.com/rdiez/JtagDue

First, I am building a GCC 8.3.0 cross-compiler toolchain for ARM Cortex-M4F on Ubuntu 18.04.4 LTS, and I am then using it to build some 
embedded software with a cross-compiling Autoconf project.

GCC introduced some time ago option -flto=jobserver in order to use the GNU 
Make jobserver when parallelising LTO builds.

I am actually building many such Autoconf projects from a top-level makefile, so using the top-level jobserver prevents overloading the 
system with too many parallel jobs. That is what the jobserver was designed for.

When doing a recursive make, you need to place a '+' character at the beginning of the recipe line in order to let GNU Make pass the 
jobserver file descriptors to the child processes. Alternatively, if you reference variable $(MAKE) inside the recipe like, you get the same 
effect. Otherwise, GNU Make does not let child processes inherit those file descriptors (or so I heard, probably with "close fd on exec").

In my top-level makefile, I added those '+' prefixes manually, so all Autoconf-generated makefiles are building in parallel but using the 
top-level jobserver.

The trouble is, invoking GCC in the linking phase with -flto=jobserver is actually a similar "recursive make" situation: GCC needs to 
inherit those jobserver file descriptors.

However, Autoconf/Automake does not generate that '+' prefix for the linking 
recipe.

Is there a way to overcome this limitation?

This will probably become a general issue in the future, as each stage could 
theoretically benefit from multi-core CPUs. See for example:

https://gcc.gnu.org/wiki/ParallelGcc

Thanks in advance,
  rdiez