[build] Overhaul <errno.h>, <signal.h> cpp symbol extraction/checking.

[build] Overhaul <errno.h>, <signal.h> cpp symbol extraction/checking.

* libguile/Makefile.am (BUILT_SOURCES): Delete cpp_err_symbols.c,
  cpp_sig_symbols.c; add cpp-E.c, cpp-SIG.c.
  (EXTRA_DIST): Delete cpp_signal.c, cpp_errno.c, cpp_err_symbols.in,
  cpp_err_symbols.c, cpp_sig_symbols.c, cpp_sig_symbols.in, cpp_cnvt.awk;
  add cpp-E.syms, cpp-E.c, cpp-SIG.syms, cpp-SIG.c.
  (error.x, posix.x): Update prereq list.
  (cpp_err_symbols.c, cpp_sig_symbols.c): Delete targets.
  (check_signals, check_errnos): Likewise.
  (.syms.c): New pattern rule.
  (chknew-E, chknew-SIG): New targets.
  (MOSTLYCLEANFILES): Delete cpp_err_symbols_here, cpp_err_symbols_diff,
  cpp_err_symbols_new, cpp_sig_symbols_here, cpp_sig_symbols_diff,
  cpp_sig_symbols_new.
* libguile/cpp-E.syms: Rename from libguile/cpp_err_symbols.in.
* libguile/cpp-SIG.syms: Rename from libguile/cpp_sig_symbols.in.
* libguile/error.c (scm_init_error): #include "libguile/cpp-E.c".
* libguile/posix.c (scm_init_posix): #include "libguile/cpp-SIG.c".
* libguile/cpp_cnvt.awk: Delete file.
* libguile/cpp_errno.c: Delete file.
* libguile/cpp_signal.c: Delete file.