setting 'char' to be signed by default since some tests fail when it's wrong…

setting 'char' to be signed by default since some tests fail when it's wrong (e.g. native compilation on ARM Linux)

cmake/OpenCVCompilerOptions.cmake

@@ -47,6 +47,9 @@ macro(add_extra_compiler_option option)
   endif()
 endmacro()
 
+# some OpenCV tests fail when 'char' is 'unsigned' by default
+add_extra_compiler_option(-fsigned-char)
+
 if(MINGW)
   # http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40838
   # here we are trying to workaround the problem