-DBUILD_PORTABLE_LINUX for static linking against opencv and libc++

This gets us back to glibc 2.27 (ubuntu 18.04) -- debian stretch would
be nicer, but the default build tools (c++1x, cmake) gets slightly more
interesting.

The output binaries seem like they could be smaller...

CMakeLists.txt

@@ -33,8 +33,17 @@ else()  # if not wasm, go find opencv. 3 or 4 should both work
 	include_directories(${OpenCV_INCLUDE_DIRS})
 endif()
 
+if(DEFINED BUILD_PORTABLE_LINUX)
+	find_package(PkgConfig REQUIRED)
+	pkg_check_modules(OPENCV4 REQUIRED opencv4)
+	link_directories(${OPENCV4_STATIC_LIBRARY_DIRS})
+
+	# statically link c++/gcc (not libc!!)
+	set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "${CMAKE_CXX_FLAGS_RELWITHDEBINFO} -static-libstdc++ -static-libgcc")
+endif()
+
 if(NOT DEFINED OPENCV_LIBS)
-	set(OPENCV_LIBS "opencv_calib3d" "opencv_imgcodecs" "opencv_imgproc" "opencv_photo" "opencv_core")
+	set(OPENCV_LIBS "opencv_calib3d" "opencv_imgcodecs" "opencv_imgproc" "opencv_photo" "opencv_core" ${OPENCV4_STATIC_LIBRARIES})
 endif()
 
 if(NOT DEFINED CPPFILESYSTEM)

package-portable-linux.sh

@@ -0,0 +1,17 @@
+#!/bin/sh
+# docker run --mount type=bind,source="$(pwd)",target="/usr/src/app" -it ubuntu:18.04
+
+cd /usr/src/app
+apt update
+apt install -y pkgconf g++ cmake
+apt install -y libgles2-mesa-dev libglfw3-dev
+
+cd opencv4/
+mkdir build-portable/ && cd build-portable/
+cmake .. -DCMAKE_BUILD_TYPE=RELEASE -DBUILD_SHARED_LIBS=OFF -DOPENCV_GENERATE_PKGCONFIG=YES -DOPENCV_FORCE_3RDPARTY_BUILD=YES
+make -j5 install
+
+cd /usr/src/app
+mkdir build-portable/ && cd build-portable/
+cmake .. -DBUILD_PORTABLE_LINUX=1
+make -j5 install