Allow complex.mk to succeed under parse_makefile

[reteps]

I have minimal understanding of this codebase, but it seems the complex.mk file fails to parse.

AUX =   README COPYING ChangeLog Makefile.in  \
        makefile.pc configure configure.in \
        tar.texinfo tar.info* texinfo.tex \
        tar.h port.h open3.h getopt.h regex.h \
        rmt.h rmt.c rtapelib.c alloca.c \
        msd_dir.h msd_dir.c tcexparg.c \
        level-0 level-1 backup-specs testpad.c
.PHONY: shar
shar: $(SRCS) $(AUX)
    shar $(SRCS) $(AUX) | compress \
          > tar-`sed -e '/version_string/!d' \
                     -e 's/[^0-9.]*\([0-9.]*\).*/\1/' \
                     -e q \
                     version.c`.shar.Z

[reteps]

Full traceback:

Traceback (most recent call last):
  File "<frozen runpy>", line 198, in _run_module_as_main
  File "<frozen runpy>", line 88, in _run_code
  File "/Users/retep/repos/pymake/pymake/pymake.py", line 559, in <module>
    main()
  File "/Users/retep/repos/pymake/pymake/pymake.py", line 556, in main
    sys.exit(_run_it(args))
             ^^^^^^^^^^^^^
  File "/Users/retep/repos/pymake/pymake/pymake.py", line 537, in _run_it
    exit_code = execute(makefile, args)
                ^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/retep/repos/pymake/pymake/pymake.py", line 497, in execute
    for rule in rulesdb.walk_tree(target):
  File "/Users/retep/repos/pymake/pymake/rules.py", line 105, in walk_tree
    yield from self.walk_tree(p)
  File "/Users/retep/repos/pymake/pymake/rules.py", line 105, in walk_tree
    yield from self.walk_tree(p)
  File "/Users/retep/repos/pymake/pymake/rules.py", line 105, in walk_tree
    yield from self.walk_tree(p)
  File "/Users/retep/repos/pymake/pymake/rules.py", line 99, in walk_tree
    rule = self.get(target)
           ^^^^^^^^^^^^^^^^
  File "/Users/retep/repos/pymake/pymake/rules.py", line 78, in get
    return self.rules[target]
           ~~~~~~~~~~^^^^^^^^
KeyError: 'regex.h'

debug.log

[reteps]

This was just an unintuitive error message -- the parsing would have succeeded if regex.h was defined.