Enable MSVC math constants for custom demo plugin (#884)

This upstreams a generalized form of RoboStack build fix patch/ros-rolling-gz-ros2-control-demos.win.patch.

Best guess rationale: the custom hardware demo plugin uses C math constants that MSVC exposes only when _USE_MATH_DEFINES is defined before including math headers. Defining it for the target on MSVC keeps the demo portable without affecting other toolchains.

Signed-off-by: Tobias Fischer <info@tobiasfischer.info>
Co-authored-by: Daisuke Nishimatsu <nishimarudai@gmail.com>

Author: Tobias-Fischer

gz_ros2_control_demos/CMakeLists.txt

@@ -86,6 +86,9 @@ PUBLIC
   rclcpp::rclcpp
   rclcpp_lifecycle::rclcpp_lifecycle
 )
+if(MSVC)
+  target_compile_definitions(gz_custom_hardware_plugins PRIVATE _USE_MATH_DEFINES)
+endif()
 
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