Don't build any generated source file at "make dist" time.

Don't build any generated source file at "make dist" time.

* srfi/Makefile.am
  (libguile_srfi_srfi_1_v_@LIBGUILE_SRFI_SRFI_1_MAJOR@_la_SOURCES,
  libguile_srfi_srfi_4_v_@LIBGUILE_SRFI_SRFI_4_MAJOR@_la_SOURCES,
  libguile_srfi_srfi_13_14_v_@LIBGUILE_SRFI_SRFI_13_14_MAJOR@_la_SOURCES,
  libguile_srfi_srfi_60_v_@LIBGUILE_SRFI_SRFI_60_MAJOR@_la_SOURCES):
  Remove `.x' files so that they're not built by "make dist".

* test-suite/standalone/Makefile.am (libtest_asmobs_la_SOURCES): Remove
  `test-asmobs-lib.x'.
  (libtest_ffi_la_SOURCES): Remove `test-ffi-lib.x'.
  (BUILT_SOURCES): Likewise.