configure: fix linking with MSVC when using --disable-optimizations

Without any optimization flags, MSVC does no dead code elimination (DCE) at
all, even for the most trivial cases. DCE is a prerequisite for building libav
correctly, otherwise there are undefined references to functions for other
architectures and disabled components.

-O1 is the minimal optimization flag for MSVC that does include DCE.

configure

@@ -3279,6 +3279,7 @@ probe_cc(){
         _DEPFLAGS='$(CPPFLAGS) $(CFLAGS) -showIncludes -Zs'
         _cflags_speed="-O2"
         _cflags_size="-O1"
+        _cflags_noopt="-O1"
         if $_cc -nologo- 2>&1 | grep -q Linker; then
             _ld_o='-out:$@'
         else