I met some problems in building the iOS project

[iosPPP]

run: cmake -DCMAKE_TOOLCHAIN_FILE=../cmake/Toolchains/ios.cmake -GXcode ..


then there are some errors from the command line:

zzzdeiMac:build-ios zzz$ cmake -DCMAKE_TOOLCHAIN_FILE=../cmake/Toolchains/ios.cmake -GXcode ..
Normal Build
-- Could NOT find SDL2 (missing: SDL2_LIBRARY SDL2_INCLUDE_DIR SDL2_SDLMAIN_LIBRARY)
-- No build type selected, default to Release
CMAKE_SYSTEM_PROCESSOR: armv7
Generating for ARMv7, Release
Generating for ARM, Release
Normal Build
-- Could NOT find SDL2 (missing: SDL2_LIBRARY SDL2_INCLUDE_DIR SDL2_SDLMAIN_LIBRARY)
CMAKE_SYSTEM_PROCESSOR: armv7
Generating for ARMv7, Release
Generating for ARM, Release
CMake Error at ext/glslang/CMakeLists.txt:338 (add_library):
add_library cannot create target "Common" because another target with the
same name already exists. The existing target is a static library created
in source directory "/Users/zzz/Desktop/ppsspp". See documentation for
policy CMP0002 for more details.


CMake Error at ext/glslang/CMakeLists.txt:384 (add_subdirectory):
add_subdirectory given source "ext" which is not an existing directory.


CMake Error at ext/glslang/CMakeLists.txt:389 (target_link_libraries):
Attempt to add link library "Ext::Snappy" to target "Common" which is not
built in this directory.


CMake Error at cmake/Modules/FindFFmpeg.cmake:147 (list):
list sub-command REMOVE_DUPLICATES requires list to be present.
Call Stack (most recent call first):
ext/glslang/CMakeLists.txt:462 (find_package)


CMake Error at /Applications/CMake.app/Contents/share/cmake-3.8/Modules/FindPackageHandleStandardArgs.cmake:137 (message):
Could NOT find FFmpeg (missing: FFmpeg_INCLUDE_avcodec
FFmpeg_LIBRARY_avcodec FFmpeg_INCLUDE_avformat FFmpeg_LIBRARY_avformat
FFmpeg_INCLUDE_avutil FFmpeg_LIBRARY_avutil FFmpeg_INCLUDE_swresample
FFmpeg_LIBRARY_swresample FFmpeg_INCLUDE_swscale FFmpeg_LIBRARY_swscale
avcodec avformat avutil swresample swscale)
Call Stack (most recent call first):
/Applications/CMake.app/Contents/share/cmake-3.8/Modules/FindPackageHandleStandardArgs.cmake:377 (_FPHSA_FAILURE_MESSAGE)
cmake/Modules/FindFFmpeg.cmake:154 (find_package_handle_standard_args)
ext/glslang/CMakeLists.txt:462 (find_package)


-- Configuring incomplete, errors occurred!
See also "/Users/zzz/Desktop/ppsspp/build-ios/CMakeFiles/CMakeOutput.log".




how can i fix these problems?