8351029: IncludeCustomExtension does not work on cygwin with source code below /home

Reviewed-by: erikj

make/PreInit.gmk

@@ -161,19 +161,19 @@ ifneq ($(SKIP_SPEC), true)
 	  ( cd $(TOPDIR) && \
 	  $(foreach spec, $(SPECS), \
 	    $(MAKE) $(MAKE_INIT_ARGS) -j 1 -f $(TOPDIR)/make/Init.gmk \
-	        SPEC=$(spec) $(MAKE_INIT_MAIN_TARGET_ARGS) \
-	        main && \
+	        SPEC=$(spec) TOPDIR_ALT=$(TOPDIR) \
+	        $(MAKE_INIT_MAIN_TARGET_ARGS) main && \
 	    $(if $(and $(COMPARE_BUILD), $(PARALLEL_TARGETS)), \
 	      $(MAKE) $(MAKE_INIT_ARGS) -f $(TOPDIR)/make/Init.gmk \
-	          SPEC=$(spec) \
+	          SPEC=$(spec) TOPDIR_ALT=$(TOPDIR) \
 	          COMPARE_BUILD="$(COMPARE_BUILD)" \
 	          pre-compare-build && \
 	      $(MAKE) $(MAKE_INIT_ARGS) -j 1 -f $(TOPDIR)/make/Init.gmk \
-	          SPEC=$(spec) $(MAKE_INIT_MAIN_TARGET_ARGS) \
+	          SPEC=$(spec) TOPDIR_ALT=$(TOPDIR) \
 	          COMPARE_BUILD="$(COMPARE_BUILD):NODRYRUN=true" \
-	          main && \
+	          $(MAKE_INIT_MAIN_TARGET_ARGS) main && \
 	      $(MAKE) $(MAKE_INIT_ARGS) -f $(TOPDIR)/make/Init.gmk \
-	          SPEC=$(spec) \
+	          SPEC=$(spec) TOPDIR_ALT=$(TOPDIR) \
 	          COMPARE_BUILD="$(COMPARE_BUILD):NODRYRUN=true" \
 	          post-compare-build && \
 	    ) \

make/PreInitSupport.gmk

@@ -250,13 +250,14 @@ endef
 # Param 1: FORCE = force generation of main-targets.gmk or LAZY = do not force.
 # Param 2: The SPEC file to use.
 define DefineMainTargets
+  SPEC_FILE := $(strip $2)
 
   # We will start by making sure the main-targets.gmk file is removed, if
   # make has not been restarted. By the -include, we will trigger the
   # rule for generating the file (which is never there since we removed it),
   # thus generating it fresh, and make will restart, incrementing the restart
   # count.
-  main_targets_file := $$(dir $(strip $2))make-support/main-targets.gmk
+  main_targets_file := $$(dir $$(SPEC_FILE))make-support/main-targets.gmk
 
   ifeq ($$(MAKE_RESTARTS), )
     # Only do this if make has not been restarted, and if we do not force it.
@@ -268,10 +269,10 @@ define DefineMainTargets
   $$(main_targets_file):
 	@( cd $$(TOPDIR) && \
 	$$(MAKE) $$(MAKE_LOG_FLAGS) -r -R -f $$(TOPDIR)/make/GenerateFindTests.gmk \
-	    -I $$(TOPDIR)/make/common SPEC=$(strip $2) )
+	    -I $$(TOPDIR)/make/common SPEC=$$(SPEC_FILE) TOPDIR_ALT=$$(TOPDIR))
 	@( cd $$(TOPDIR) && \
 	$$(MAKE) $$(MAKE_LOG_FLAGS) -r -R -f $$(TOPDIR)/make/Main.gmk \
-	    -I $$(TOPDIR)/make/common SPEC=$(strip $2) NO_RECIPES=true \
+	    -I $$(TOPDIR)/make/common SPEC=$$(SPEC_FILE) TOPDIR_ALT=$$(TOPDIR) NO_RECIPES=true \
 	    $$(MAKE_LOG_VARS) \
 	    create-main-targets-include )
 

make/common/MakeFileStart.gmk

@@ -47,7 +47,7 @@ endif
 # We need spec.gmk to get $(TOPDIR)
 include $(SPEC)
 
-THIS_MAKEFILE := $(patsubst make/%,%,$(patsubst $(TOPDIR)/%,%,$(THIS_MAKEFILE_PATH)))
+THIS_MAKEFILE := $(patsubst make/%,%,$(patsubst $(TOPDIR_ALT)/make/%,%,$(patsubst $(TOPDIR)/%,%,$(THIS_MAKEFILE_PATH))))
 
 ifeq ($(LOG_FLOW), true)
   $(info :Enter $(THIS_MAKEFILE))

make/common/MakeIncludeStart.gmk

@@ -29,7 +29,7 @@
 
 # Get the next to last word (by prepending a padding element)
 THIS_INCLUDE_PATH := $(word $(words ${MAKEFILE_LIST}),padding ${MAKEFILE_LIST})
-THIS_INCLUDE := $(patsubst $(TOPDIR)/make/%,%,$(THIS_INCLUDE_PATH))
+THIS_INCLUDE := $(patsubst $(TOPDIR_ALT)/make/%,%,$(patsubst $(TOPDIR)/make/%,%,$(THIS_INCLUDE_PATH)))
 
 # Print an indented message, also counting the top-level makefile as a level
 ifneq ($(INCLUDE_GUARD_$(THIS_INCLUDE)), true)