build: detect drm_color_ctm_3x4 in system libdrm headers

The amdgpu plugin's local amdgpu_drm.h defines struct
drm_color_ctm_3x4 guarded by #ifndef HAVE_DRM_COLOR_CTM_3X4,
but HAVE_DRM_COLOR_CTM_3X4 was never defined by the build
system. On systems where libdrm headers already provide this
struct (e.g. Alpine with libdrm 2.4.131), clang rejects the
redefinition as a hard error, while gcc silently accepts
identical struct redefinitions as an extension.

Add a feature test that checks whether struct drm_color_ctm_3x4
is available in the system drm_mode.h and pass
-DHAVE_DRM_COLOR_CTM_3X4 when it is, so the local fallback
definition is skipped.

Assisted-by: Claude Code (claude-opus-4-6):2025-09-03
Signed-off-by: Adrian Reber <areber@redhat.com>

Makefile.config

@@ -26,6 +26,11 @@ endif
 
 ifeq ($(call pkg-config-check,libdrm),y)
         export CONFIG_AMDGPU := y
+        LIBDRM_CFLAGS	:= $(shell $(PKG_CONFIG) --cflags libdrm)
+        ifeq ($(call try-cc,$(FEATURE_TEST_DRM_COLOR_CTM_3X4),,$(LIBDRM_CFLAGS)),true)
+                export CONFIG_HAS_DRM_COLOR_CTM_3X4 := y
+                FEATURE_DEFINES	+= -DHAVE_DRM_COLOR_CTM_3X4
+        endif
         $(info Note: Building with amdgpu_plugin.)
 else
         $(info Note: Building without amdgpu_plugin.)

scripts/feature-tests.mak

@@ -156,6 +156,18 @@ int main(void)
 }
 endef
 
+define FEATURE_TEST_DRM_COLOR_CTM_3X4
+
+#include <drm_mode.h>
+
+int main(void)
+{
+	struct drm_color_ctm_3x4 ctm = {};
+
+	return (int)ctm.matrix[0];
+}
+endef
+
 define FEATURE_TEST_NO_LIBC_RSEQ_DEFS
 
 #ifdef __has_include