xgettext: Simplify code that emits diagnostics with error number.

xgettext: Simplify code that emits diagnostics with error number.

* autogen.sh (GNULIB_MODULES_TOOLS_FOR_SRC): Add xstrerror.
(GNULIB_MODULES_LIBGETTEXTPO_OTHER): Add strerror_r-posix.
* gettext-tools/src/x-csharp.c: Include xstrerror.h.
(phase2_getc): Invoke 'xstrerror' and 'if_error' instead of 'error'.
* gettext-tools/src/x-javascript.c: Include xstrerror.h.
(phase2_getc): Invoke 'xstrerror' and 'if_error' instead of 'error'.
* gettext-tools/src/x-python.c: Include xstrerror.h.
(phase2_getc): Invoke 'xstrerror' and 'if_error' instead of 'error'.