Make words() work on a list of non-strings

words() attempts to join all the elements of a list
and then resplit them to more closely match make.
But sometimes, like when calling words() on a product
variable, not all of the elements are strings. In
that case, just return the list unchanged.

Bug: 267407943
Test: ./out/rbcrun ./build/make/tests/run.rbc
Change-Id: I738d0c86c8935f446807cc79623f796e8cae3c01

tests/run.rbc

@@ -46,6 +46,11 @@ assert_eq("", rblf.mkstrip(" \n \t    "))
 assert_eq("a b c", rblf.mkstrip("  a b   \n  c \t"))
 assert_eq("1", rblf.mkstrip("1 "))
 
+assert_eq(["a", "b"], rblf.words("a b"))
+assert_eq(["a", "b", "c"], rblf.words(["a b", "c"]))
+# 1-tuple like we use in product variables
+assert_eq(["a b", ("c",)], rblf.words(["a b", ("c",)]))
+
 assert_eq("b1 b2", rblf.mksubst("a", "b", "a1 a2"))
 assert_eq(["b1", "x2"], rblf.mksubst("a", "b", ["a1", "x2"]))