win32: Bump libsigc++ dependency. Improve Cross-Compile doc

Older libsigc++ doesn't build with newer MingW headers, but the
latest one does and is compatible.

Update the WIN32_CROSS_COMPILE.txt to note that the gcc-mingw32
variant of MingW successfully compiles things that work these
days.
amf-tools: Fix compilation on win32 with newer MingW

stdio.h is the correct header to include on any platform to get
fopen and friends.