This patch is courtesy of BLFS.

--- ninja-1.9.0.old/src/ninja.cc
+++ ninja-1.9.0/src/ninja.cc
@@ -221,6 +221,11 @@
 
 /// Choose a default value for the -j (parallelism) flag.
 int GuessParallelism() {
+  int j = 0;
+  char *jobs = getenv ("NINJAJOBS");
+  if (jobs != NULL) j = atoi (jobs);
+  if (j > 0) return j;
+
   switch (int processors = GetProcessorCount()) {
   case 0:
   case 1: