Add missing include for string.h.
Initialize port streams in SCM_initialize.

GCC no longer considers stdin, stdout, stderr to be const expressions
for the purpose of static initialization.
Fix case typo: test-Scheme -> test-scheme.
Export certain Make vars so they are available in scheme.

These changes allow test.chap10e.c to compile, but it segfaults
immediatedly on launch.
Use ~S directive when writing Scheme strings to C strings.
Copy warning about continuations from start.chap6f to test.chap6f.
Remove chap6f from BROKEN file and BROKEN_TESTS.