[slave.mk] Define SPACE variable differently such that the line doesn't end in whitespace (#1676)

Author: jleveque

slave.mk

@@ -12,8 +12,8 @@ GUID = $(shell id -g)
 
 .SECONDEXPANSION:
 
-SPACE :=
-SPACE +=
+NULL :=
+SPACE := $(NULL) $(NULL)
 
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 ## General definitions