libstdc++ testsuite fixes, 1

[Benjamin Kosnik]

thanks Zack. 

> If you build using a relative pathname to get from the object to the
> source directory, testsuite_flags --build-includes will generate -I
> switches which are relative to the top level of the libstdc++ build
> directory.  runtest executes one level below (in the testsuite
> directory), therefore all the paths are wrong, and the test suite
> can't find any headers and blows up horribly.

kay. I didn't see this, but that just means I am configuring/building 
differently. What's your setup?

> This patch corrects the problem.  I chose to edit the result of
> testsuite_flags from libstdc++-v3-dg.exp, rather than change
> testsuite_flags, because there are other things (mkcheck.in, e.g.)
> which use testsuite_flags and may expect it to produce its old
> results.

Hmm. testsuite_flags should be giving non-relative output, so it needs to 
get fixed at that point (which shouldn't prohibit mkcheck.in from working)

also the includes need to include the ext directory for tests using hash_map.

-benjamin