Makefile: use -j8, allow manual PARALLEL_MAKE val

Set default -j to 8: even on a machine with less cores, using -j8 isn't really
going to slow down the build. On a machine with eight cores, -j8 is going to
speed up significantly.

By using '?=', allow passing a different -jN as PARALLEL_MAKE from the
enviroment.

Change-Id: Idf6a5505742b30d3cb8b80ae98b4e2ad4a4a38e7
diff --git a/Makefile b/Makefile
index 3b5995a..8cdbbec 100644
--- a/Makefile
+++ b/Makefile
@@ -1,6 +1,6 @@
 SUBDIRS=bsc bsc-nat bts ggsn_tests gprs_gb hlr lapdm mgw msc selftest sgsn sysinfo
 
-PARALLEL_MAKE:=-j4
+PARALLEL_MAKE ?= -j8
 
 # This master makefile allows you to do things like
 # 	make clean	(remove all generated binary, c++ and symlinks)