Various compatibility fixes and improvements to msvcc.sh.

* Don't try to mix incompatible optimization flags in debug builds.
* Workaround ax_cc_maxopt.m4 not supporting MSVC and change -O3 to -O2.
* Fix MSVC warning by properly passing linker flags to compiler.
* Make msvcc.sh return 1 if invalid command line options are used rather than silently eating them.
* Add more comments.
Merge pull request #107 from rvandermeulen/msvcc

Various compatibility fixes and improvements to msvcc.sh.