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.