configure.ac: add -fsigned-char to make sure char is signed
configure.ac: add -fsigned-char to make sure char is signed

on systems like the Raspberry Pi char is unsigned by default but our
code expects char to be signed. So we should be good with -fsigned-char

Tatsh/libimobiledevice,libimobiledevice/libimobiledevice,kalev/libimobiledevice,libimobiledevice/libimobiledevice,libimobiledevice-win32/libimobiledevice,libimobiledevice-win32/libimobiledevice,kalev/libimobiledevice,libimobiledevice-win32/libimobiledevice,Tatsh/libimobiledevice,libimobiledevice-win32/libimobiledevice,Tatsh/libimobiledevice,kalev/libimobiledevice,upbit/libimobiledevice,patweb/libimobiledevice,upbit/libimobiledevice,upbit/libimobiledevice,patweb/libimobiledevice,patweb/libimobiledevice,Tatsh/libimobiledevice