Merge #18928: build: don't pass -w when building for Windows

89fea68 build: don't pass -w when building for Windows (fanquake)

Pull request description:

  This has been around since the introduction of autotools. However at
  this point I'm not sure we'd ever 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.

ACKs for top commit:
  hebasto:
    ACK 89fea68

Tree-SHA512: 2b5bdef7fff5c87b28199f5822cab3cdf600c90c01a40db5cd85053eef5dcb5816e2e97ff61a30ff94b4f0c6cb7be22beaef34d82235bdf05ff9da865d40b381

Author: fanquake

configure.ac

@@ -578,9 +578,6 @@ 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