Makefile: fix parallel compilation using inkscape

Inkscape does DBus accesses at startup to look for other running
instances. The feature fails quite often for some obscure reason.
Disable it using (undocumented) SELF_CALL environment variable.

Code doing it in Inkscape:
https://gitlab.com/inkscape/inkscape/-/blob/a77b160f/src/inkscape-application.cpp#L557

They set it for their child extensions:
https://gitlab.com/inkscape/inkscape/-/blob/0b4d3151/src/extension/extension.cpp#L529-530

Found in a bug comment:
https://gitlab.com/inkscape/inkscape/-/issues/4716#note_1898150983

Computation of INKSCAPE_IS_NEW must be done using the same variable as
we do recursive make calls.

Before:

⟩ make clean
⟩ make -j20 full-linux-kernel-slides.pdf
make: *** [Makefile:271: <...PATH-TO-TRAINING-MATERIALS...>/out/./common/sd-card.pdf] Error 134
make: *** Waiting for unfinished jobs....
make: *** [Makefile:271: <...PATH-TO-TRAINING-MATERIALS...>/out/./common/qemu-logo.pdf] Error 134
make: *** [Makefile:269: <...PATH-TO-TRAINING-MATERIALS...>/out/./common/strace-c-output.pdf] Error 134
make: *** [Makefile:269: <...PATH-TO-TRAINING-MATERIALS...>/out/./agenda/qemu-logo.pdf] Error 134
make: *** [Makefile:269: <...PATH-TO-TRAINING-MATERIALS...>/out/./common/gdb-vs-gdbserver.pdf] Error 134
make: *** [Makefile:269: <...PATH-TO-TRAINING-MATERIALS...>/out/./common/logo-cc.pdf] Error 134
⟩ # note that failing targets is random and changes on each call

After:

⟩ make clean
⟩ make -j20 full-linux-kernel-slides.pdf
⟩ # works without errors

Signed-off-by: Théo Lebrun <[email protected]>

Makefile

@@ -1,11 +1,13 @@
 # See the README file for usage instructions
 
-INKSCAPE = inkscape
+# SELF_CALL avoids checking other instances, which fails with --jobs > 1.
+# https://gitlab.com/inkscape/inkscape/-/issues/4716#note_1898150983
+INKSCAPE = SELF_CALL=true inkscape
 PDFLATEX = xelatex
 DIA      = dia
 EPSTOPDF = epstopdf
 
-INKSCAPE_IS_NEW = $(shell inkscape --version | grep -q "^Inkscape 1" && echo YES)
+INKSCAPE_IS_NEW = $(shell $(INKSCAPE) --version | grep -q "^Inkscape 1" && echo YES)
 
 ifeq ($(INKSCAPE_IS_NEW),YES)
 INKSCAPE_PDF_OPT = -o