introduce --with-rootprefix=DIR

Configure the location of the rootfs directories and use it
to find (/usr)/lib/modules and (/usr)/lib/modprobe.d.
diff --git a/Makefile.am b/Makefile.am
index 79e12db..46c025a 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -6,6 +6,7 @@
 AM_CPPFLAGS = \
 	-include $(top_builddir)/config.h \
 	-I$(top_srcdir)/libkmod \
+	-DROOTPREFIX=\""$(rootprefix)"\" \
 	-DSYSCONFDIR=\""$(sysconfdir)"\" \
 	-DLIBEXECDIR=\""$(libexecdir)"\"