strstr, strcasestr: replace on platforms with broken memchr

strstr, strcasestr: replace on platforms with broken memchr

* modules/strstr: Split into...
* modules/strstr-simple: ...new module that does not care about
performance, but does care about glibc bug.
* m4/strstr.m4 (gl_FUNC_STRSTR): Split...
(gl_FUNC_STRSTR_SIMPLE): ...into new macro, which replaces strstr
if platform memchr is broken, per Debian bug 521737.
* m4/strcasestr.m4 (gl_FUNC_STRCASESTR_SIMPLE): React to broken
memchr.
* m4/memchr.m4 (gl_FUNC_MEMCHR): Only expand once.
* doc/posix-functions/strstr.texi (strstr): Document the fix.
* doc/glibc-functions/strcasestr.texi (strcasestr): Likewise.
* modules/mountlist (Depends-on): Add strstr-simple.
* modules/gen-uni-tables (Depends-on): Likewise.
* modules/argz (Depends-on): Add strstr.

Signed-off-by: Eric Blake <ebb9@byu.net>