enable full GCC Stack-smashing protection for all OSes

- change our hardening options to use -fstack-protector-all even for
  Windows builds, as we recently switched to a newer compiler suite
- also removes an obsolete workaround for GCC 4.5
  (https://bugs.launchpad.net/ubuntu/+source/gcc-4.5/+bug/691722), which
  required to first set -fno-stack-protector, before -fstack-protector-all

configure.ac

@@ -284,6 +284,7 @@ AX_CHECK_LINK_FLAG([[-Wl,--large-address-aware]], [LDFLAGS="$LDFLAGS -Wl,--large
 
 if test x$use_hardening != xno; then
   AX_CHECK_COMPILE_FLAG([-Wstack-protector],[HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -Wstack-protector"])
+  AX_CHECK_COMPILE_FLAG([-fstack-protector-all],[HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -fstack-protector-all"])
   AX_CHECK_COMPILE_FLAG([-fPIE],[HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -fPIE"])
 
   AX_CHECK_PREPROC_FLAG([-D_FORTIFY_SOURCE=2],[
@@ -299,10 +300,6 @@ if test x$use_hardening != xno; then
   AX_CHECK_LINK_FLAG([[-Wl,-z,now]], [LDFLAGS="-Wl,-z,now"])
 
   if test x$TARGET_OS != xwindows; then
-    # -fstack-protector-all can produce broken binaries with mingw
-    AX_CHECK_COMPILE_FLAG([-fno-stack-protector],[HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -fno-stack-protector"])
-    AX_CHECK_COMPILE_FLAG([-fstack-protector-all],[HARDENED_CXXFLAGS="$HARDENED_CXXFLAGS -fstack-protector-all"])
-
     # -pie will link successfully with MinGW, but it's unsupported and leads to undeterministic binaries
     AX_CHECK_LINK_FLAG([[-pie]], [HARDENED_LDFLAGS="$HARDENED_LDFLAGS -pie"])
   fi