external project: invokes make with -j$(nproc), incompatible with jobservers

[thesamesam]

If meson is already running under a jobserver (in MAKEFLAGS) when an external project is built, the make call will ignore it, as it always passes the equivalent of -j$(nproc) instead:

meson/mesonbuild/scripts/externalproject.py

Lines 50 to 51 in 14143be

	if self.supports_jobs_flag():
	make_cmd.append(f'-j{determine_worker_count()}')

We should check if MAKEFLAGS (and perhaps GNUMAKEFLAGS to not make the problem worse, though ninja and some others won't look at that) contains --jobserver-auth before adding -j ourselves.

You can see this with e.g (assuming the test fails to make it clear):

$ export MAKEFLAGS="--jobserver-auth=fifo:/dev/steve
$  ./run_single_test.py "./test cases/common/230 external project"
[...]

Build targets in project: 3

test external project undefined

  User defined options
    backend: ninja
    libdir : lib
    prefix : /usr

Found ninja-1.13.2 at /usr/bin/ninja
ninja: Jobserver mode detected: -j32 -l32 --jobserver-auth=fifo:/dev/steve
ninja explain: deps for 'libsomelib.so.p/func.c.o' are missing
[1/6] Compiling C object libsomelib.so.p/func.c.o
ninja explain: libsomelib.so.p/func.c.o is dirty
[2/6] Linking target libsomelib.so
ninja explain: deps for 'libfoo/libfoo.stamp' are missing
ninja explain: libsomelib.so is dirty
[2/6] Generating external project libfoo/libfoo
make: warning: -j32 forced in submake: resetting jobserver mode. 
ninja explain: libsomelib.so is dirty

One can reproduce this with a make invocation that just runs meson too.