include: Declare getopt function on old GNU libc

On GNU libc <= 2.25, <unistd.h> includes <getopt.h> with __need_getopt macro
defined.  That <getopt.h> is intended to be a part of GNU libc but
<unistd.h> actually includes include/getopt.h in this project.

If HAVE_DECL_GETOPT is defined to 1 and include/getopt.h is included from
GNU libc's <unistd.h>, declaration of getopt is suppressed, causing errors
on getopt callers.  This issue is possibly hidden so long because there are
not so many true getopt callers in Binutils, GDB and GCC.  Still, this issue
needs to be fixed for following components:

-   Binutils: gprofng
    (not currently affected due to the configuration script but will be)
-   GDB (sim): M32C simulator
-   GDB (sim): RL78 simulator

To avoid not defining proper getopt declaration, we have to check
__need_getopt macro to detect this include path.  With this commit, even if
HAVE_DECL_GETOPT is 1, getopt is declared if:

-   The standard C library is GNU libc and
-   __need_getopt macro is defined (<unistd.h> includes <getopt.h> to
    declare getopt function).

include/ChangeLog:

	* getopt.h: Detect special include path on GNU libc 2.25 or older
	to prevent not declaring getopt function when necessary.

include/getopt.h

@@ -104,7 +104,14 @@ struct option
    declaration without arguments.  If it is 0, we checked and failed
    to find the declaration so provide a fully prototyped one.  If it
    is 1, we found it so don't provide any declaration at all.  */
-#if !HAVE_DECL_GETOPT
+/* On GNU libc <= 2,25, <unistd.h> includes <getopt.h> with __need_getopt
+   macro defined.  That <getopt.h> is intended to be a part of GNU libc
+   but <unistd.h> actually includes THIS getopt.h.  If HAVE_DECL_GETOPT is
+   defined to 1 and this file is included from GNU libc's <unistd.h>,
+   declaration of getopt is suppressed, causing errors on getopt callers.
+   To avoid not defining proper getopt declaration, we have to check
+   __need_getopt macro when built with GNU libc to detect this include path.  */
+#if !HAVE_DECL_GETOPT || (defined (__GNU_LIBRARY__) && defined (__need_getopt))
 #if defined (__GNU_LIBRARY__) || defined (HAVE_DECL_GETOPT)
 /* Many other libraries have conflicting prototypes for getopt, with
    differences in the consts, in unistd.h.  To avoid compilation