Additional LDFLAGS needed by AIX

[Benjamin Kosnik]

> 	I need some way of inserting an additional LDFLAG when linking
> shared libraries on AIX, similar to OPT_LDFLAGS and SECTION_LDFLAGS.  I
> thought about abusing one of those two existing LDFLAGS, but
> configure.target is included before those to flags are cleared in
> GLIBCPP_CHECK_LINKER_FEATURES:
> 
>   SECTION_LDFLAGS=''
>   OPT_LDFLAGS=''

Hmm. I don't think GLIBCPP_CHECK_LINKER_FEATURES should be clearing 
either of these. In general, it looks like libstdc++-v3's efforts to 
outsmart the linker have failed. 

> Should OPT_LDFLAGS or SECTION_LDFLAGS not clear any previous value?

Right.