Refactoring: Introduce distinct host and target rpath var setters.

`HOST_RPATH_VAR$(1)_T_$(2)_H_$(3)` and `TARGET_RPATH_VAR$(1)_T_$(2)_H_$(3)`
both match the format of the old `RPATH_VAR$(1)_T_$(2)_H_$(3)` (which
is still being set the same way that it was before, to one of either
HOST/TARGET depending on what stage we are building).  Namely, the format
is <XXX>_RPATH_VAR = "<LD_LIB_PATH_ENVVAR>=<COLON_SEP_PATH_ENTRIES>"

This should be a pure refactoring apart from the introduction of the
two new make environment variables.

Author: pnkfelix

mk/main.mk

@@ -357,13 +357,19 @@ endef
 define SREQ_CMDS
 
 ifeq ($$(OSTYPE_$(3)),apple-darwin)
-  RPATH_VAR$(1)_T_$(2)_H_$(3) := \
+  HOST_RPATH_VAR$(1)_T_$(2)_H_$(3) := \
       DYLD_LIBRARY_PATH="$$$$DYLD_LIBRARY_PATH:$$(CURDIR)/$$(HLIB$(1)_H_$(3))"
+  TARGET_RPATH_VAR$(1)_T_$(2)_H_$(3) := \
+      DYLD_LIBRARY_PATH="$$$$DYLD_LIBRARY_PATH:$$(CURDIR)/$$(TLIB1_T_$(2)_H_$(CFG_BUILD))"
 else
-  RPATH_VAR$(1)_T_$(2)_H_$(3) := \
+  HOST_RPATH_VAR$(1)_T_$(2)_H_$(3) := \
       LD_LIBRARY_PATH="$$$$LD_LIBRARY_PATH:$$(CURDIR)/$$(HLIB$(1)_H_$(3))"
+  TARGET_RPATH_VAR$(1)_T_$(2)_H_$(3) := \
+      LD_LIBRARY_PATH="$$$$LD_LIBRARY_PATH:$$(CURDIR)/$$(TLIB1_T_$(2)_H_$(CFG_BUILD))"
 endif
 
+RPATH_VAR$(1)_T_$(2)_H_$(3) := $$(HOST_RPATH_VAR$(1)_T_$(2)_H_$(3))
+
 # Pass --cfg stage0 only for the build->host part of stage0;
 # if you're building a cross config, the host->* parts are
 # effectively stage1, since it uses the just-built stage0.
@@ -379,13 +385,7 @@ ifeq ($(1),0)
 ifneq ($(strip $(CFG_BUILD)),$(strip $(3)))
 CFGFLAG$(1)_T_$(2)_H_$(3) = stage1
 
-ifeq ($$(OSTYPE_$(3)),apple-darwin)
-  RPATH_VAR$(1)_T_$(2)_H_$(3) := \
-      DYLD_LIBRARY_PATH="$$$$DYLD_LIBRARY_PATH:$$(CURDIR)/$$(TLIB1_T_$(2)_H_$(CFG_BUILD))"
-else
-  RPATH_VAR$(1)_T_$(2)_H_$(3) := \
-      LD_LIBRARY_PATH="$$$$LD_LIBRARY_PATH:$$(CURDIR)/$$(TLIB1_T_$(2)_H_$(CFG_BUILD))"
-endif
+RPATH_VAR$(1)_T_$(2)_H_$(3) := $$(TARGET_RPATH_VAR$(1)_T_$(2)_H_$(3))
 endif
 endif