Fix the %_smp_mflags macro so it works in both TW and SLE16

Unfortunately SLE16 uses old rpm version 4.18 which does not
support %{getncpus:proc} (added in 4.19).

We need to compute the value manually. :-/

rust/package/agama.spec

@@ -15,6 +15,21 @@
 # Please submit bugfixes or comments via https://bugs.opensuse.org/
 #
 
+# Require at least 1.3GB RAM per each parallel job (the size is in MB)
+%global _smp_tasksize_proc 1300
+
+# Redefine the _smp_mflags macro so it takes the amount of RAM into account
+#
+# Unfortunately SLE16 contains older rpm v4.18 so we need to calculate that manually :-/
+# TODO: use this instead when rpm in SLE16 is updated to a newer version (4.19+):
+#       %%define _smp_mflags "-j%%{getncpus:proc}"
+%define _smp_mflags %([ -z "$RPM_BUILD_NCPUS" ] \\\
+  && RPM_BUILD_NCPUS="`/usr/bin/getconf _NPROCESSORS_ONLN`"; \\\
+  ram_kb="$(awk '/MemTotal/ {print $2}' /proc/meminfo)"; \\\
+  ncpus_max=$(("$ram_kb"/1024/%_smp_tasksize_proc)); \\\
+  if [ -n "$ncpus_max" ] && [ "$ncpus_max" -gt 0 ] && [ "$RPM_BUILD_NCPUS" -gt "$ncpus_max" ]; then RPM_BUILD_NCPUS="$ncpus_max"; fi; \\\
+  if [ "$RPM_BUILD_NCPUS" -gt 1 ]; then echo "-j$RPM_BUILD_NCPUS"; fi)
+
 Name:           agama
 #               This will be set by osc services, that will run after this.
 Version:        0