package/opencv3: drop support for protobuf

The protobuf support breaks the build, as protobuf includes
libabseil-cpp headers, which now require C++14. opencv3 doesn't have
any ENABLE_CXX14 option, so for the time being, disable protobuf
support until someone bothers enough to fix this up.

While we suspect a libabseil-cpp version bump to be responsible for
the issue, we are not 100% sure. However, the issue is definitely
present in Buildroot 2025.02.x, as it can be reproduced using the
following defconfig:

BR2_aarch64=y
BR2_TOOLCHAIN_EXTERNAL=y
BR2_TOOLCHAIN_EXTERNAL_BOOTLIN=y
BR2_PACKAGE_OPENCV3=y
BR2_PACKAGE_OPENCV3_LIB_SHAPE=y
BR2_PACKAGE_OPENCV3_LIB_STITCHING=y
BR2_PACKAGE_OPENCV3_LIB_SUPERRES=y
BR2_PACKAGE_OPENCV3_LIB_TS=y
BR2_PACKAGE_OPENCV3_LIB_VIDEOSTAB=y
BR2_PACKAGE_OPENCV3_WITH_PROTOBUF=y

Fixes:

  https://autobuild.buildroot.net/results/39432e7746e6bc5224592a7d2f744ca992bd529a/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Julien Olivain <ju.o@free.fr>

Config.in.legacy

@@ -146,6 +146,13 @@ endif
 
 comment "Legacy options removed in 2026.02"
 
+config BR2_PACKAGE_OPENCV3_WITH_PROTOBUF
+	bool "opencv3 protobuf support removed"
+	select BR2_LEGACY
+	help
+	  Support for OpenCV 3 protobuf support has been removed as it
+	  was no longer compatible with recent versions of protobuf.
+
 config BR2_PACKAGE_RESIPROCATE_APPS
 	bool "resiprocate apps support option removed"
 	select BR2_LEGACY

package/opencv3/Config.in

@@ -278,18 +278,6 @@ config BR2_PACKAGE_OPENCV3_WITH_PNG
 	help
 	  Use shared libpng from the target system.
 
-config BR2_PACKAGE_OPENCV3_WITH_PROTOBUF
-	bool "protobuf support"
-	depends on BR2_PACKAGE_PROTOBUF_ARCH_SUPPORTS # protobuf
-	depends on BR2_TOOLCHAIN_GCC_AT_LEAST_8 # protobuf
-	select BR2_PACKAGE_PROTOBUF
-	help
-	  Use shared protobuf from the target system.
-
-comment "protobuf support needs a toolchain w/ gcc >= 8"
-	depends on BR2_PACKAGE_PROTOBUF_ARCH_SUPPORTS
-	depends on !BR2_TOOLCHAIN_GCC_AT_LEAST_8
-
 config BR2_PACKAGE_OPENCV3_WITH_TIFF
 	bool "tiff support"
 	select BR2_PACKAGE_TIFF

package/opencv3/opencv3.mk

@@ -223,6 +223,7 @@ OPENCV3_CONF_OPTS += \
 	-DWITH_OPENEXR=OFF \
 	-DWITH_OPENNI2=OFF \
 	-DWITH_OPENNI=OFF \
+	-DWITH_PROTOBUF=OFF \
 	-DWITH_UNICAP=OFF \
 	-DWITH_VA=OFF \
 	-DWITH_VA_INTEL=OFF \
@@ -282,17 +283,6 @@ else
 OPENCV3_CONF_OPTS += -DWITH_PNG=OFF
 endif
 
-ifeq ($(BR2_PACKAGE_OPENCV3_WITH_PROTOBUF),y)
-# protobuf needs c++11 (since 3.6.0)
-OPENCV3_CONF_OPTS += \
-	-DENABLE_CXX11=ON \
-	-DPROTOBUF_UPDATE_FILES=ON \
-	-DWITH_PROTOBUF=ON
-OPENCV3_DEPENDENCIES += protobuf
-else
-OPENCV3_CONF_OPTS += -DWITH_PROTOBUF=OFF
-endif
-
 ifeq ($(BR2_PACKAGE_OPENCV3_WITH_QT5),y)
 OPENCV3_CONF_OPTS += -DWITH_QT=5
 OPENCV3_DEPENDENCIES += qt5base