[v3] tweak -ffunction-sections configure test

[Benjamin Kosnik]

> Is the reason for disabling it due to the warning causing spurious
> FAILs in the testsuite (or halting the build when -Werror is turned
> on), or because there's actually some problem with the library built
> with -ffunction-sections?

There is a problem with debugging a library built with
"-ffunction-sections" on this target. If you don't have to debug, then
things will be ok.

-benjamin