Bug#1041076: dolphin-emu FTBFS with imgui 1.89.6

[Adrian Bunk]

Source: dolphin-emu
Version: 5.0-19368+dfsg-2
Severity: serious
Tags: ftbfs

https://tests.reproducible-builds.org/debian/rb-pkg/unstable/amd64/dolphin-emu.html

...
In file included from /build/1st/dolphin-emu-5.0-19368+dfsg/Externals/implot/implot/implot_internal.h:39,
                 from /build/1st/dolphin-emu-5.0-19368+dfsg/Externals/implot/implot/implot_items.cpp:26:
/usr/include/imgui/imgui_internal.h:101:2: error: #error Please '#define IMGUI_DEFINE_MATH_OPERATORS' _BEFORE_ including imgui.h!
  101 | #error Please '#define IMGUI_DEFINE_MATH_OPERATORS' _BEFORE_ including imgui.h!
      |  ^~~~~
In file included from /build/1st/dolphin-emu-5.0-19368+dfsg/Externals/implot/implot/implot_internal.h:39,
                 from /build/1st/dolphin-emu-5.0-19368+dfsg/Externals/implot/implot/implot.cpp:126:
/usr/include/imgui/imgui_internal.h:101:2: error: #error Please '#define IMGUI_DEFINE_MATH_OPERATORS' _BEFORE_ including imgui.h!
  101 | #error Please '#define IMGUI_DEFINE_MATH_OPERATORS' _BEFORE_ including imgui.h!
      |  ^~~~~
...