Allow to force to do not use -j as make argument

Close #828 (hopefully)

Makefile.rule

@@ -112,7 +112,10 @@ NO_AFFINITY = 1
 # NO_PARALLEL_MAKE = 1
 
 # Force number of make jobs. The default is the number of logical CPU of the host.
-# This is particularly useful when using distcc
+# This is particularly useful when using distcc.
+# A negative value will disable adding a -j flag to make, allowing to use a parent
+# make -j value. This is usefull to call OpenBLAS make from an other project
+# makefile
 # MAKE_NB_JOBS = 2
 
 # If you would like to know minute performance report of GotoBLAS.

getarch.c

@@ -1013,7 +1013,12 @@ int main(int argc, char *argv[]){
 #endif
 
 #ifdef MAKE_NB_JOBS
+  #if MAKE_NB_JOBS > 0
     printf("MAKE += -j %d\n", MAKE_NB_JOBS);
+  #else
+    // Let make use parent -j argument or -j1 if there
+    // is no make parent
+  #endif
 #elif NO_PARALLEL_MAKE==1
     printf("MAKE += -j 1\n");
 #else