Fix various minor issues with rt-tests build system

The issues fixed are :

 * Remove the automatic NUMA detection from the host
   architecture. This is broken when doing cross-compilation. One can
   still set NUMA=1 if NUMA support is desired.

 * Expand the CFLAGS provided through the environment instead of
   overriding it.

 * Provide a HASPYTHON variable to tell whether the target system has
   Python or not. Otherwise, the build system simply tests whether
   Python is available on the host. The PYLIB variable is also changed
   so that it can be overriden from the environment, in order to
   provide the correct Python module location for the target.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

Index: rt-tests/Makefile
===================================================================
--- rt-tests.orig/Makefile
+++ rt-tests/Makefile
@@ -14,15 +14,11 @@
 mandir	?= $(prefix)/share/man
 srcdir	?= $(prefix)/src
 
-machinetype = $(shell uname -m | \
-    sed -e 's/i.86/i386/' -e 's/mips.*/mips/' -e 's/ppc.*/powerpc/')
-ifneq ($(filter x86_64 i386 ia64 mips powerpc,$(machinetype)),)
-NUMA 	:= 1
-endif
-
-CFLAGS = -D_GNU_SOURCE -Wall -Wno-nonnull -Isrc/include
+override CFLAGS += -D_GNU_SOURCE -Wall -Wno-nonnull -Isrc/include
 
-PYLIB  := $(shell python -c 'import distutils.sysconfig;  print distutils.sysconfig.get_python_lib()')
+ifeq ($(HASPYTHON),1)
+PYLIB ?= $(shell python -c 'import distutils.sysconfig;  print distutils.sysconfig.get_python_lib()')
+endif
 
 ifndef DEBUG
 	CFLAGS	+= -O2