build: add common.mk

Rather than specifying common makefile variables move
them all to a common.mk.

Signed-off-by: Spencer Oliver <ntfreak@users.sourceforge.net>

src/Makefile.am

@@ -1,3 +1,5 @@
+include $(top_srcdir)/common.mk
+
 SUBDIRS = \
 	jtag \
 	helper \
@@ -29,12 +31,6 @@ noinst_HEADERS = \
 	hello.h \
 	openocd.h
 
-
-# set the include path found by configure
-AM_CPPFLAGS = \
-	-I$(top_srcdir)/src \
-	-I$(top_builddir)/src
-
 libopenocd_la_CPPFLAGS = -DPKGBLDDATE=\"`date +%F-%R`\"
 
 # banner output includes RELSTR appended to $VERSION from the configure script