handle 'iconv's that define EILSEQ to ENOENT

handle 'iconv's that define EILSEQ to ENOENT

We're currently pulling gnulib's errno module as a dependency of some
other module.  That provides an errno.h that defines EILSEQ to a
distinct value if the system's errno.h doesn't define it already.

However, GNU iconv does this:

 /* Get errno declaration and values. */
 #include <errno.h>
 /* Some systems, like SunOS 4, don't have EILSEQ. Some systems, like BSD/OS,
    have EILSEQ in a different header.  On these systems, define EILSEQ
    ourselves. */
 #ifndef EILSEQ
 #define EILSEQ @EILSEQ@
 #endif

That's in:

 http://git.savannah.gnu.org/cgit/libiconv.git/tree/include/iconv.h.in

The "different header" mentioned is wchar.h.  This is handled in:

 http://git.savannah.gnu.org/cgit/libiconv.git/tree/m4/eilseq.m4

which defines @EILSEQ@ to ENOENT if EILSEQ isn't found in either
errno.h or wchar.h.

So if iconv sets errno to EILSEQ on such system's, it's really setting
it to ENOENT.  And when we check for EILSEQ, we're checking for
gnulib's value.  The result is we won't detect the error correctly.

As we dropped support for both SunOS 4 or old BSD/OS, maybe we don't
need to care about the wchar.h issue anymore.  Still, AFAICS, gnulib's
m4/errno_h.m4 doesn't know that EILSEQ may be defined in wchar.h, and
so on such systems, ISTM gnulib ends up defining an incompatible
EILSEQ itself, but I think that should be fixed on the gnulib side, by
making it extract the EILSEQ value out of the system's wchar.h, like
GNU iconv does.

So that leaves handling the case of gnulib making up a EILSEQ value,
which we take as meaning the system really doesn't really define it,
which will be the same systems GNU iconv sets errno to ENOENT instead
of EILSEQ.

Looking at glibc's iconv it seems that ENOENT is never used there.
It seems it's safe to always treat ENOENT the same as EILSEQ.

The current EILSEQ definition under PHONY_ICONV is obviously stale as
gnulib garantees there's always a EILSEQ defined.

Tested on x86_64 Fedora 20.

gdb/
2014-11-14  Pedro Alves  <palves@redhat.com>

	* charset.c [PHONY_ICONV && !EILSEQ] (EILSEQ): Don't define.
	[!PHONY_ICONV] (gdb_iconv): New function.
	[!PHONY_ICONV] (iconv): Redefine to gdb_iconv.