Replace nproc with getconf _NPROCESSORS_ONLN

``getconf _NPROCESSORS_ONLN`` is available in both Linux and macOS, but
``nproc`` is not. This change prevents the build from breaking again if
macOS ever upgrades its `make` to version 4 or above.

Author: Naitreey

Doc/Makefile

@@ -242,7 +242,7 @@ dist-pdf:
 	# ($$ is needed to escape the $; https://www.gnu.org/software/make/manual/make.html#Basics-of-Variable-References)
 	-sed -i 's/: all-$$(FMT)/:/' build/latex/Makefile
 	if [ -n "$(filter output-sync,$(value .FEATURES))" ]; then \
-		(cd build/latex; $(MAKE) clean && $(MAKE) --jobs=$$((`nproc`+1)) --output-sync LATEXMKOPTS='-quiet' all-pdf && $(MAKE) FMT=pdf zip bz2) \
+		(cd build/latex; $(MAKE) clean && $(MAKE) --jobs=$$((`getconf _NPROCESSORS_ONLN`+1)) --output-sync LATEXMKOPTS='-quiet' all-pdf && $(MAKE) FMT=pdf zip bz2) \
 	else \
 		(cd build/latex; $(MAKE) clean && $(MAKE) --jobs=$$((`getconf _NPROCESSORS_ONLN`+1)) LATEXMKOPTS='-quiet' all-pdf && $(MAKE) FMT=pdf zip bz2) \
 	fi