Emscripten wasm side module library

[danoli3]

As Emscripten libraries require executable code rather than static this fixes issues for WASM using side module.
.wasm extension target

Can be interpreted as .a with this executable byte code

This can be used for patching your own libpng frameworks.
Emscripten comes with a em port however versions are not always up to date with latest changes so this is convenient for cmake / developers / frameworks.

I think also we can assume libpng is being compiled by Emscripten via emmake

Example Cmake commands:

mkdir -p build_$TYPE
	    cd build_$TYPE
	    rm -f CMakeCache.txt *.a *.o *.wasm

	    ZLIB_ROOT="$LIBS_ROOT/zlib/"
	    ZLIB_INCLUDE_DIR="$LIBS_ROOT/zlib/include"
	    ZLIB_LIBRARY="$LIBS_ROOT/zlib/lib/$TYPE/zlib.wasm"

	    $EMSDK/upstream/emscripten/emcmake cmake .. \
	    	${DEFS} \
	    	-DCMAKE_TOOLCHAIN_FILE=$EMSDK/upstream/emscripten/cmake/Modules/Platform/Emscripten.cmake \
	    	-DCMAKE_C_STANDARD=17 \
	    	-DEMSCRIPTEN=ON \
			-DCMAKE_CXX_STANDARD=17 \
			-DCMAKE_CXX_STANDARD_REQUIRED=ON \
			-DCMAKE_CXX_FLAGS="-DUSE_PTHREADS=1 -std=c++17 -Wno-implicit-function-declaration -frtti -fPIC ${FLAG_RELEASE}" \
			-DCMAKE_C_FLAGS="-DUSE_PTHREADS=1 -std=c17 -Wno-implicit-function-declaration -frtti -fPIC ${FLAG_RELEASE}" \
			-DCMAKE_CXX_EXTENSIONS=OFF \
			-DCMAKE_INSTALL_PREFIX=Release \
			-DZLIB_ROOT=${ZLIB_ROOT} \
			-DZLIB_LIBRARY=${ZLIB_LIBRARY} \
			-DZLIB_INCLUDE_DIR=${ZLIB_INCLUDE_DIR} \
			-DZLIB_INCLUDE_DIRS=${ZLIB_INCLUDE_DIR} \
			-DCMAKE_BUILD_TYPE=Release \
			-DBUILD_SHARED_LIBS=ON \
			-DPNG_EXECUTABLES=OFF \
			-DPNG_BUILD_ZLIB=OFF
	    cmake --build . --target install --config Release
	    cd ..

[ctruta]

This is not an actual review comment, but rather, a question:

Is pngfix needed for the Emscripten build?

I just pushed a new commit, see aa95dee. I'm still building the third-party contributed tools for now, if only not to break the downstream package maintainers' workflows. However, I want to decouple those tools from the main library build, including pngfix, eventually. They should be built independently from libpng, in a manner that allows us to verify the correctness of a libpng installation.

One thing that you may or may not want to consider is adding EMSCRIPTEN besides ANDROID and IOS at line 72:

libpng/CMakeLists.txt

Line 72 in aa95dee

if (ANDROID OR IOS)

[jbowler]

@ctruta: won't fix. No reply from OP