[PATCH] Fix the --enable-shared build

[Sanjoy Das]

> Passing -fno-devirtualize to broken gcc versions.

Won't that affect performance?  The in-source workaround only fixes
the issue affecting us, and seems lesser impact that a global
-fno-devirtualize.  The source code impact is likely to be greater as
well.

Moreover, I could not come up with a test program that links
successfully with clang++ / older g++ but breaks with g++ 4.9.  The
example on the gcc bug tracker doesn't successfully link with clang
because of a missing definition for PassNameParser::PassNameParser and
main, and adding those in passes the test for g++ 4.9 as well.  This
means we'd probably have to hard-code the g++ version number, and not
use AC_LINK_IFELSE.  We could also parse the error messages produced
when linking, but that seems fragile.

-- Sanjoy