Fix unused variable warning when !LIBMESH_HAVE_MPI
Fix GCC 4.8 'possibly uninitialized' variable warning.
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

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.

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!

Refs #386.
Add a configure test to see if clang supports the address sanitizer
options we want to use.
Use perl (when available) to determine the clang version number.

Detect if the Clang compiler is from Apple.

Move AC_PATH_PROG before LIBMESH_SET_COMPILERS
Try actually running a test program compiled with address sanitizer
flags to make sure that there is no runtime error, such as dynamic
libraries not being found.
Ran bootstrap