build: don't pass -w when building for Windows

This has been around since the introduction of autotools. However at
this point I'm not sure we'd every want to suppress all warnings when
performing a build, and given that CXX FLAGS will have been overriden
when cross-compiling for Windows (using depends), this would rarely,
if-ever be used anyways.

From https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html:
-w

    Inhibit all warning messages.

configure.ac

@@ -576,9 +576,6 @@ case $host in
      fi
 
      CPPFLAGS="$CPPFLAGS -D_MT -DWIN32 -D_WINDOWS -DBOOST_THREAD_USE_LIB -D_WIN32_WINNT=0x0601"
-     if test "x$CXXFLAGS_overridden" = "xno"; then
-       CXXFLAGS="$CXXFLAGS -w"
-     fi
 
      dnl libtool insists upon adding -nostdlib and a list of objects/libs to link against.
      dnl That breaks our ability to build dll's with static libgcc/libstdc++/libssp. Override