patch to check ld

[Benjamin Kosnik]

> Shouldn't that go into the specs?  That seems to me to be the right
> place to do that sort of black magic instead of requiring users to
> manually play with that tricky flag, IMHO.

The precedent has already been set with -Wl,--gc-sections and such. 

In effect, libstdc++-v3 is setting linker flags that it notices are 
available, and activating them if they are helpful or advantageous for 
c++ libraries. 

IMHO, these flags should be the default in the linker itself, but they are 
currently not.

I think adding all this stuff as part of autoconf (there is a nice macro 
for checking and enableing in acinclude.m4) seems a nice way to do 
this.

-benjamin