When second expansion is enabled make substitutes all instances of a '%' in each prerequisite of a static pattern rule.

$ cat makefile
.SECONDEXPANSION:
all: hello.x
hello.x: %.x: $$(wordlist 1, 99, %_%_%.1 %_%_%.2); $(info $@ from $^)
$ /usr/bin/make
make: *** No rule to make target 'hello_hello_hello.1', needed by 'hello.x'.  Stop.

This patch
1. Causes make to substitute only the first instance of '%' in each prerequisite of a static pattern rule when second expansion is enabled.

So, the example above results in

$ ~/src/gmake/make/l64/make
make: *** No rule to make target 'hello_%_%.1', needed by 'hello.x'.  Stop.

2. Adds a set of tests to verify that stem substitution is the same whether second expansion is enabled or not.

3. Fixes a memory leak in expand_deps