Add %constrain_build macro

Adds a simple macro which limits the CPU count which is used in the
various build-related macro.  Using this macro at the beginning of the
specfile to limit the CPU count directly, or to set the CPU count based
on the amount of memory in the system and a "job size".

The default action if no options are provided is to limit builds to a
single CPU.

macros.build-constraints

@@ -1,5 +1,47 @@
 # Macros to constrain resource use during the build process
 
+# Changes _smp_build_ncpus depending on various factors
+#
+# -c cpus  constrains the CPU count to "cpus"
+# -m mem   constrains the CPU count to the total amount of memory in the system
+#          (in megabytes) divided by "mem", rounded down
+#
+# If no options are passed, sets _smp_build_ncpus to 1.
+# _smp_build_ncpus will never be raised, only lowered.
+%constrain_build(c:m:) %{lua:
+  local mem_limit = math.tointeger(opt.m)
+  local cpu_limit = math.tointeger(opt.c)
+  local current_cpus = math.tointeger(macros._smp_build_ncpus)
+  local constrained_cpus = current_cpus
+
+  -- Parse meminfo to find the total amount of memory in the system
+  local function getmem()
+    local mem = 0
+    for l in io.lines('/proc/meminfo') do
+      if l:sub(1, 9) == "MemTotal:" then
+        mem = math.tointeger(string.match(l, "MemTotal:%s+(%d+)"))
+        break
+      end
+    end
+    return mem
+  end
+
+  if (not cpu_limit and not mem_limit) then
+    cpu_limit = 1
+  end
+
+  if cpu_limit ~= nil then
+    constrained_cpus = math.min(cpu_limit, constrained_cpus)
+  end
+  if mem_limit ~= nil then
+    local mem_total = getmem(verbose)
+    local limit = math.max(1, mem_total // (mem_limit * 1024))
+    constrained_cpus = math.min(constrained_cpus, limit)
+  end
+
+  macros._smp_build_ncpus = constrained_cpus
+}
+
 # outputs build flag overrides to be used in conjunction with
 # %%make_build, %%cmake_build etc.
 #