[libstdc++] Build system cleanup 8/n

[Benjamin Kosnik]

> But it is actually checked for in both various crossconfig and native 
> (line 121 configure.ac). It—like most other headers—is not checked in 
> newlib crossbuilds, which seems wrong. Regardless this patch seems safe.

Ok. I didn't catch the configure.ac line when I did the review of your
other patch, sorry.

> I actually don't see the reason for why any AC_CHECK_HEADERS would be 
> split into host or target specific checks as checking if a header file 
> exists is quick and wouldn't hurt if it doesn't exist, but I'll queue 
> that cleanup to the next round after acconfig.h makes its long awaited 
> departure.

Agreed about checking headers. In general, I think the goal should be to
try and move as much as possible into one place only, non-link checks
that happen for all builds, regardless of native/cross.

However, this can wait for a bit.

-benjamin