[v3] tweak -ffunction-sections configure test

[Benjamin Kosnik]

> This test is still problematic, at least on MinGW (and thus probably
> Cygwin but I haven't checked it yet.)  On a cross from current HEAD I
> see this:
> 
> configure:16661: checking for g++ that supports -ffunction-sections
> -fdata-sections
> configure:16680:  /b/mingw-43-dw2-cross/./gcc/xgcc -shared-libgcc
> -B/b/mingw-43-dw2-cross/./gcc -nostdinc++
> -L/b/mingw-43-dw2-cross/i686-pc-mingw32/libstdc++-v3
> /src -L/b/mingw-43-dw2-cross/i686-pc-mingw32/libstdc++-v3/src/.libs
> -L/b/mingw-43-dw2-cross/i686-pc-mingw32/winsup/mingw
> -L/b/mingw-43-dw2-cross/i686-pc-mingw32
> /winsup/w32api/lib -isystem /s/c/winsup/mingw/include -isystem
> /s/c/winsup/w32api/include
> -B/i/mingw-43-dw2-cross/i686-pc-mingw32/bin/ -B/i/mingw-43-dw2-cross/i
> 686-pc-mingw32/lib/ -isystem
> /i/mingw-43-dw2-cross/i686-pc-mingw32/include -isystem
> /i/mingw-43-dw2-cross/i686-pc-mingw32/sys-include
> -L/b/mingw-43-dw2-cross/./
> ld -c -g -Werror -ffunction-sections -fdata-sections  conftest.cc >&5
> cc1plus: warnings being treated as errors
> conftest.cc:1: error: -ffunction-sections may affect debugging on some
> targets
> configure:16686: $? = 1
> configure: failed program was:
> [...]
> configure:16718: result: no

This is the behavior I was expecting on mingw32. I'm not quite sure why
you think this is in error.

The end result should be that -ffunction-sections is not used when
compiling the files in libstdc++-v3/src on this target.

-benjamin