[build] enable concurrent compilation (#2888)

before
real    2m18.077s
user    1m55.516s
sys     0m21.719s

after
real    0m30.610s
user    2m34.688s
sys     0m28.359s

Author: ahorek

Makefile

@@ -19,6 +19,7 @@ ifndef COVERAGE
   CFLAGS   += -O2
   CXXFLAGS += -O2
   LDFLAGS  += -O2
+  MAKEFLAGS += "-j 0"
 else
   CFLAGS   += -O1 -fno-omit-frame-pointer
   CXXFLAGS += -O1 -fno-omit-frame-pointer