backport of RELDIR (needed for https://trac.osgeo.org/grass/changeset/62444)

git-svn-id: https://svn.osgeo.org/grass/grass/branches/releasebranch_7_0@62545 15284696-431f-4ddb-bdfa-cd5b030d7da7

include/Make/Grass.make

@@ -19,6 +19,8 @@
 #
 #########################################################################
 
+RELDIR		:= $(subst $(GRASS_HOME)/,,$(CURDIR))
+
 # GRASS global directories and constants
 # platform specific dirs
 ARCH_DISTDIR	= $(GRASS_HOME)/dist.$(ARCH)