Silence -Werror,-Wundef in external/boost

> [CXX] vcl/unx/gtk3_kde5/gtk3_kde5_folderpicker.cxx
> In file included from vcl/unx/gtk3_kde5/gtk3_kde5_folderpicker.cxx:20:
> In file included from vcl/unx/gtk3_kde5/gtk3_kde5_folderpicker.hxx:30:
> In file included from vcl/unx/gtk3_kde5/gtk3_kde5_filepicker_ipc.hxx:29:
> In file included from workdir/UnpackedTarball/boost/boost/process/child.hpp:22:
> In file included from workdir/UnpackedTarball/boost/boost/process/detail/execute_impl.hpp:24:
> workdir/UnpackedTarball/boost/boost/process/detail/posix/executor.hpp:446:5: error: 'BOOST_POSIX_HAS_VFORK' is not defined, evaluates to 0 [-Werror,-Wundef]
> #if BOOST_POSIX_HAS_VFORK
>     ^

under --enable-gtk3-kde5

Change-Id: Ib4648d1337a493c35b35897dc71c5e971c6eb214

external/boost/include/boost/process/child.hpp

+/* generated by bin/gen-boost-headers, do not edit! */
+#pragma once
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wpragmas"  /* first! for GCC */
+#pragma GCC diagnostic ignored "-Wunknown-warning-option" // second! for Clang 5
+#pragma GCC diagnostic ignored "-Wdelete-non-virtual-dtor"
+#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
+#pragma GCC diagnostic ignored "-Wextra"
+#pragma GCC diagnostic ignored "-Wignored-qualifiers"
+#pragma GCC diagnostic ignored "-Wimplicit-fallthrough"
+#pragma GCC diagnostic ignored "-Winvalid-constexpr"
+#pragma GCC diagnostic ignored "-Wmicrosoft-unqualified-friend"
+#pragma GCC diagnostic ignored "-Wnon-virtual-dtor"
+#pragma GCC diagnostic ignored "-Wplacement-new"
+#pragma GCC diagnostic ignored "-Wreturn-type"
+#pragma GCC diagnostic ignored "-Wshadow"
+#pragma GCC diagnostic ignored "-Wstrict-aliasing"
+#pragma GCC diagnostic ignored "-Wtautological-constant-out-of-range-compare"
+#pragma GCC diagnostic ignored "-Wtype-limits"
+#pragma GCC diagnostic ignored "-Wundef"
+#pragma GCC diagnostic ignored "-Wunused-local-typedefs"
+#pragma GCC diagnostic ignored "-Wunused-macros"
+#pragma GCC diagnostic ignored "-Wunused-parameter"
+#pragma GCC diagnostic ignored "-Wunused-variable"
+#include_next <boost/process/child.hpp>
+#pragma GCC diagnostic pop