参考吧规贴我现在是这么写的：
cmake_minimum_required(VERSION 3.9)
project(untitled1 C)
set(CMAKE_C_STANDARD 99)
set(SDL2_INC_DIR D:\\CObject\\library\\SDL2\\include)
set(SDL2_LIB_DIR D:\\CObject\\library\\SDL2\\lib)
set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -lmingw32 -lSDL2main -mwindows ")
include_directories(${SDL2_INC_DIR})
link_directories(${SDL2_LIB_DIR})
link_libraries(SDL2)
add_executable(untitled1 main.c)
target_link_libraries(untitled1 SDL2)
。。。有没有错误啊！

总是报错呢：
D:/CObject/library/SDL2/lib/SDL2main.lib(Win32/Release/SDL_windows_main.obj):(.text[_main]+0x0): multiple definition of `main'
e:/maingw/bin/../lib/gcc/mingw32/6.3.0/../../../libmingw32.a(main.o):(.text.startup+0x0): first defined here
collect2.exe: error: ld returned 1 exit status
CMakeFiles\untitled1.dir\build.make:96: recipe for target 'untitled1.exe' failed
mingw32-make.exe[3]: *** [untitled1.exe] Error 1
mingw32-make.exe[2]: *** [CMakeFiles/untitled1.dir/all] Error 2
CMakeFiles\Makefile2:66: recipe for target 'CMakeFiles/untitled1.dir/all' failed
CMakeFiles\Makefile2:78: recipe for target 'CMakeFiles/untitled1.dir/rule' failed
mingw32-make.exe[1]: *** [CMakeFiles/untitled1.dir/rule] Error 2
Makefile:117: recipe for target 'untitled1' failed
mingw32-make.exe: *** [untitled1] Error 2

Cmake的文件能用mingw32_make.exe？我一直以为mingw32_make.exe只能makefile呢。

报错信息里面写了main函数重定义。
在SDL2main库里面已经有了一个main函数，你自己的main函数其实并不是程序入口。
两个解决方法：
自己的main函数必须写成 int main(int argc, char* argv[]) 的标准形式，或者自己的程序中#undef main