[PATCH] configure.ac: use -Wno-unused-command-line-argument if supported

[Austin English]

This avoids ~9000 warnings like:
austin at gcc113:~$ grep  'warning: argument unused during compilation'
without-patch.build.log | sort -u
clang-6.0: warning: argument unused during compilation: '-I ../atl'
[-Wunused-command-line-argument]
clang-6.0: warning: argument unused during compilation: '-I
../d3dcompiler_43' [-Wunused-command-line-argument]
clang-6.0: warning: argument unused during compilation: '-I ../d3dx11_43'
[-Wunused-command-line-argument]
clang-6.0: warning: argument unused during compilation: '-I ../d3dx9_36'
[-Wunused-command-line-argument]
clang-6.0: warning: argument unused during compilation: '-I
../../../include/msvcrt' [-Wunused-command-line-argument]
clang-6.0: warning: argument unused during compilation: '-I
../../include/msvcrt' [-Wunused-command-line-argument]
clang-6.0: warning: argument unused during compilation: '-I
../../../include' [-Wunused-command-line-argument]
clang-6.0: warning: argument unused during compilation: '-I ../../include'
[-Wunused-command-line-argument]
clang-6.0: warning: argument unused during compilation: '-I ../include'
[-Wunused-command-line-argument]
clang-6.0: warning: argument unused during compilation: '-I ../msvcm80'
[-Wunused-command-line-argument]
clang-6.0: warning: argument unused during compilation: '-I ../msvcp90'
[-Wunused-command-line-argument]
clang-6.0: warning: argument unused during compilation: '-I ../msvcrt'
[-Wunused-command-line-argument]
clang-6.0: warning: argument unused during compilation: '-I
/usr/include/dbus-1.0' [-Wunused-command-line-argument]
clang-6.0: warning: argument unused during compilation: '-I
/usr/include/freetype2' [-Wunused-command-line-argument]
clang-6.0: warning: argument unused during compilation: '-I
/usr/include/libpng12' [-Wunused-command-line-argument]
clang-6.0: warning: argument unused during compilation: '-I
/usr/include/libxml2' [-Wunused-command-line-argument]
clang-6.0: warning: argument unused during compilation: '-I
/usr/include/p11-kit-1' [-Wunused-command-line-argument]
clang-6.0: warning: argument unused during compilation: '-I
/usr/lib/aarch64-linux-gnu/dbus-1.0/include'
[-Wunused-command-line-argument]
clang-6.0: warning: argument unused during compilation: '-I ../wscript'
[-Wunused-command-line-argument]
clang-6.0: warning: argument unused during compilation: '-I ./..'
[-Wunused-command-line-argument]
clang-6.0: warning: argument unused during compilation: '-I ../..'
[-Wunused-command-line-argument]
clang-6.0: warning: argument unused during compilation: '-I .'
[-Wunused-command-line-argument]
clang-6.0: warning: argument unused during compilation: '-I ../xinput1_3'
[-Wunused-command-line-argument]
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