Various compatibility fixes and improvements to msvcc.sh.
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.

plicease/libffi,rth7680/libffi,bgarrels/libffi,johne53/MB3Libffi,Distrotech/libffi,rth7680/libffi,rth7680/libffi,Distrotech/libffi,joshtriplett/libffi,s1341/libffi,joshtriplett/libffi,Distrotech/libffi,plicease/libffi,s1341/libffi,rth7680/libffi,s1341/libffi,Pan7/libffi,nmav/libffi,biosbits/libffi,Pan7/libffi,bgarrels/libffi,Distrotech/libffi,biosbits/libffi,bgarrels/libffi,Pan7/libffi,nmav/libffi,johne53/MB3Libffi,johne53/MB3Libffi,plicease/libffi,joshtriplett/libffi,nmav/libffi,Pan7/libffi,plicease/libffi,nmav/libffi,biosbits/libffi,bgarrels/libffi,johne53/MB3Libffi,joshtriplett/libffi,s1341/libffi,johne53/MB3Libffi,biosbits/libffi