package/opencv4: fix uclibc build

Add BR2_PACKAGE_OPENCV4_LIB_DNN to fix the following uclibc build
failure raised since bump to version 4.5.5 in commit
b6922313937712a89841b58d3b355edd53754bca and
https://github.com/opencv/opencv/commit/1feb3838b51c0b27b0f3e64f02358ff5ef3111b8:

/home/giuliobenetti/autobuild/run/instance-3/output-1/build/opencv4-4.5.5/modules/dnn/src/layers/elementwise_layers.cpp: In member function 'float cv::dnn::RoundFunctor::calculate(float) const':
/home/giuliobenetti/autobuild/run/instance-3/output-1/build/opencv4-4.5.5/modules/dnn/src/layers/elementwise_layers.cpp:1260:43: error: 'fegetround' is not a member of 'std'
 1260 |         int old_rounding_direction = std::fegetround();
      |                                           ^~~~~~~~~~
/home/peko/autobuild/instance-0/output-1/build/opencv4-4.5.5/modules/dnn/src/layers/elementwise_layers.cpp:1261:14: error: 'fesetround' is not a member of 'std'
 1261 |         std::fesetround(FE_TONEAREST);
      |              ^~~~~~~~~~
/home/peko/autobuild/instance-0/output-1/build/opencv4-4.5.5/modules/dnn/src/layers/elementwise_layers.cpp:1261:25: error: 'FE_TONEAREST' was not declared in this scope
 1261 |         std::fesetround(FE_TONEAREST);
      |                         ^~~~~~~~~~~~

Indeed, uclibc pretends to implement fenv but it raises build failures
on numerous architectures (or1k, sh4, xtensa, mipsel, arm, arc, riscv64,
etc.)

Fixes:
 - http://autobuild.buildroot.org/results/a6dd3c10acfbf6bc1af867d770cf0ce926c6ce84

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/opencv4/Config.in

@@ -30,6 +30,17 @@ config BR2_PACKAGE_OPENCV4_LIB_CALIB3D
 	  Include opencv_calib3d (camera calibration and 3d
 	  reconstruction) module into the OpenCV build.
 
+config BR2_PACKAGE_OPENCV4_LIB_DNN
+	bool "dnn"
+	# dnn needs fenv.h which is not provided by uclibc
+	depends on !BR2_TOOLCHAIN_USES_UCLIBC
+	help
+	  Include opencv_dnn (Deep Neural Networks) module into the
+	  OpenCV build.
+
+comment "dnn needs a glibc or musl toolchain"
+	depends on BR2_TOOLCHAIN_USES_UCLIBC
+
 config BR2_PACKAGE_OPENCV4_LIB_FEATURES2D
 	bool "features2d"
 	select BR2_PACKAGE_OPENCV4_LIB_FLANN

package/opencv4/opencv4.mk

@@ -92,6 +92,7 @@ OPENCV4_CONF_OPTS += \
 	-DBUILD_opencv_apps=OFF \
 	-DBUILD_opencv_calib3d=$(if $(BR2_PACKAGE_OPENCV4_LIB_CALIB3D),ON,OFF) \
 	-DBUILD_opencv_core=ON \
+	-DBUILD_opencv_dnn=$(if $(BR2_PACKAGE_OPENCV4_LIB_DNN),ON,OFF) \
 	-DBUILD_opencv_features2d=$(if $(BR2_PACKAGE_OPENCV4_LIB_FEATURES2D),ON,OFF) \
 	-DBUILD_opencv_flann=$(if $(BR2_PACKAGE_OPENCV4_LIB_FLANN),ON,OFF) \
 	-DBUILD_opencv_highgui=$(if $(BR2_PACKAGE_OPENCV4_LIB_HIGHGUI),ON,OFF) \