* Makefile.am (EXTRA_DIST): add errnos_cnvt.awk, errnos.default,

* Makefile.am (EXTRA_DIST): add errnos_cnvt.awk, errnos.default,
errnos_get.c.
Add a rule to generate errnos.c from errnos.
* error.c (scm_init_error): include errnos.c.
* errnos_cnvt.awk: new file, converts the list of errno codes to
C expressions.
* errnos_get.c: new file.
* errnos.default: new file, contains errnos to try if they can't
be extracted from errno.h.
* configure.in: if using GCC, try and extract errno codes from
errno.h.
Added AC_PROG_AWK.