[v3] 28811, 54482

[Benjamin De Kosnik]

Fixes for various shared/static/pic issues, see SUBJECT. 

tested x86_64/linux
tested x86_64/linux --with-pic
tested x86_64/linux --without-pic*
tested x86_64/linux --disable-shared
tested x86_64/linux --disable-static

earlier versions tested on cross arm-eabisim

Most of these do what you'd expect. The default is -prefer-pic. Even if
the exact route is sideways and full of libtool mystery....Configuring
without pic, ie, --without-pic, fails earlier (in liblto-plugin), and
is otherwise ignored in non-libtool libraries like libgcc, but can be
simulated and ends up creating an incorrect shared library, as might be
expected. Seems like --disable-shared a better option for that scenario.

I added some notes on the src build machinery to the manual. 

There's a lot of fighting libtool here. There's a patch to allow a less tortured mechanism
for setting libtool's compile (not link) output, for both shared and
static compiles. See:
http://gcc.gnu.org/ml/gcc-patches/2012-09/msg00340.html

This seems like a good idea.

This patch can be considered (c). With the completion of (a), and (b),
the libtool variable overrides in libstdc++/configure.ac can be removed.
If this tortured example proves the case for either a or b to the
libtool maintainers then I'll have accomplished something useful today.

-benjamin
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