fix CPPFLAGS when building outside of source directory

mpi/Makefile.am

@@ -42,7 +42,7 @@ NVCC ?= nvcc
 endif
 
 LIBS = $(top_builddir)/src/libstarpu.la @LIBS@
-AM_CPPFLAGS = -I$(top_srcdir)/include/ -I$(top_srcdir)/mpi/ -I$(top_srcdir)/src/  -I$(top_srcdir)/examples/
+AM_CPPFLAGS = -I$(top_srcdir)/include/ -I$(top_srcdir)/mpi/ -I$(top_srcdir)/src/  -I$(top_srcdir)/examples/ -I$(top_builddir)/src -I$(top_builddir)/include
 
 lib_LTLIBRARIES = libstarpumpi.la