-linux_n_cpus = `grep '^processor' /proc/cpuinfo | wc -l`
-
-MAKE_PARALLEL_JOBS = \
- -j $(shell \
- if [ -f /proc/cpuinfo ] ; then \
- expr 2 '*' $(linux_n_cpus) ; \
- else \
- echo 1 ; \
- fi)
-
+# /proc/cpuinfo does not exist on platforms without a /proc and on some
+# platforms, notably inside containers, it has no content. In those cases
+# we assume there's 1 processor; we use 2*ncpu for the -j option.
+# NB: GNU Make 4.2 will let us use '$(file </proc/cpuinfo)' to both test
+# for file presence and content; for now this will have to do.
+MAKE_PARALLEL_JOBS = -j $(if $(shell [ -f /proc/cpuinfo ] && head /proc/cpuinfo), \
+ $(shell expr 2 '*' $$(grep -c ^processor /proc/cpuinfo)), 2)