[build] Properly track GCC language configure fragments

[build] Properly track GCC language configure fragments

The 'gcc/configure' script sources all 'gcc/*/config-lang.in' files, but fails
to emit such dependency information into the build machinery.  That means,
currently, when something gets changed in a 'gcc/*/config-lang.in' file, this
is not noticed, and doesn't propagate through the build machinery.

Handling of configure fragments is modelled in the same way as it already
exists for Makefile fragments.

	gcc/
	* Makefile.in (LANG_CONFIGUREFRAGS): Define.
	(config.status): Use/depend on it.
	* configure.ac (all_lang_configurefrags): Track, 'AC_SUBST'.
	* configure: Regenerate.

Backport from trunk r278035.

From-SVN: r278036