Fix configure test for unordered_set.

This had no chance of passing because it was not valid C++.
Remove old references to aclocal.m4 in comments.

aclocal.m4 is now generated by automake...

Refs #386.
Fix AC_MSG_RESULT typo.
Add (yet another) GCC version case.

Is this really the best way to do this?
Adding --enable-sanitize="opt dbg" configure option

With clang, but the situation does not seem to be as simple as it is
for GCC.  Just because you have clang/llvm version 3.5, it doesn't
mean your compiler was built with address sanitizer support!

Remarks:
.) We can't directly set sanitize options in e.g. CXXFLAGS_OPT in
   LIBMESH_SET_CXX_FLAGS, since CXXFLAGS_OPROF inherits CXXFLAGS_OPT in
   LIBMESH_SET_METHODS, and we might not want it to inherit the sanitizer
   flags.
.) Use perl (when available) to determine the clang version number.
   Move AC_PATH_PROG(PERL) up before LIBMESH_SET_COMPILERS.
   Also detect if the Clang compiler is from Apple.
.) Add a configure test which runs a test program to see if the
   compiler actually supports the address sanitizer flags we intend to
   use.  Actually running a test program helps ensure there are no
   runtime error, such as dynamic libraries not being found.
.)

Refs #386.
Ran bootstrap