message: build: always attempt to enable targeted sse42 cxxflags
  This avoids a counter-intuitive drop in performance when manually adjusting the
  flags.
  This is a port of Core "build: fix missing sse42 in depends builds"
diff: @@ -298,6 +298,8 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
  AX_CHECK_COMPILE_FLAG([-Wunused-local-typedef],[CXXFLAGS="$CXXFLAGS -Wno-unused-local-typedef"],,[[$CXXFLAG_WERROR]])
  AX_CHECK_COMPILE_FLAG([-Wdeprecated-register],[CXXFLAGS="$CXXFLAGS -Wno-deprecated-register"],,[[$CXXFLAG_WERROR]])
  AX_CHECK_COMPILE_FLAG([-Wthread-safety-analysis],[CXXFLAGS="$CXXFLAGS -Wthread-safety-analysis"],,[[$CXXFLAG_WERROR]])
  +fi
  +
  # Check for optional instruction set support. Enabling these does _not_ imply that all code will
  # be compiled with them, rather that specific objects/libs may use them after checking for runtime
  # compatibility.
  @@ -305,8 +307,6 @@ if test "x$CXXFLAGS_overridden" = "xno"; then
  AX_CHECK_COMPILE_FLAG([-msse4.1],[[SSE42_CXXFLAGS="-msse4.1"]],,[[$CXXFLAG_WERROR]])
  AX_CHECK_COMPILE_FLAG([-mavx -mavx2],[[AVX2_CXXFLAGS="-mavx -mavx2"]],,[[$CXXFLAG_WERROR]])
  -fi
  -
  TEMP_CXXFLAGS="$CXXFLAGS"
  CXXFLAGS="$CXXFLAGS $SSE42_CXXFLAGS"
  AC_MSG_CHECKING(for assembler crc32 support)
  