Some minor changes in Makefile and doxygen.cfg

Changes in doxygen.cfg are to reflect current source files
.
Change in Makefile are for convenience and I think they are worth committing to the trunk.
1) asan target - build compiler with Address Sanitizer instrumentation, it's nice verification tool. We have no memory bugs, which asan can catch now.
2) clang - build with clang instead of gcc.
3) debug - build debug version of ispc.