Bugfixes for Mac OS X, etc.

Hello! Here are some bugs that cropped up for me as was tinkering with Avian.
I'm building on Mac OS X 10.9 with Clang 3.3.1-RC2. These apply to both the master and v0.7.1 branches.

**makefile**:
- clang++ instead of clang: This errors when building the LZMA C files with 'clang -std=c++11': ```error: invalid argument '-std=c++11' not allowed with 'C/ObjC'```.

- don't use ```-g3``` on Darwin: -g3 is typically ignored on Darwin, but for some reason, this is causing libLTO to fail when linking the bootimage-generator. Re-added for other platforms a couple lines below; plus ```-g``` on darwin if we're not building for speed or size.

- also sandwiched in there, ```-O4``` [is no longer special-cased](http://clang.llvm.org/docs/ReleaseNotes.html#new-compiler-flags) by Clang, and in fact warns/errors loudly about it. Replaced with equivalent ```-O3 -flto```.

- Basically the same issue as the first, although since the rule in place here specifies only ```.c``` files, just use cc instead of cxx.

- Test utility won't build without this when building with LZMA.

**x86.h**
- When building for x86_64, these are the wrong headers (and in fact I get an error that they can't be found). Is there a reason to _not_ use the regular forwarding headers? 

**classpath-openjdk.cpp**
- Unused variable warning/error when building with openjdk= but not openjdk-src.

**compile.cpp**
- Unused variable warning/error.

**main.cpp**
- Clang tells me that this cannot be inlined.



... thats it for now ;)

Best, G.