compilation error when compiling casbin lib

Hi All,

I am trying to build the project and getting compilation error:

```
# create build directory
oem@maxim:~/work/casbin-CMake-setup$ mkdir build
oem@maxim:~/work/casbin-CMake-setup$ cd build/

# generate makefiles
oem@maxim:~/work/casbin-CMake-setup/build$ cmake ..
-- The CXX compiler identification is GNU 9.3.0
-- The C compiler identification is GNU 9.3.0
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++ - skipped
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working C compiler: /usr/bin/cc - skipped
-- Detecting C compile features
-- Detecting C compile features - done
-- Configuring done
-- Generating done
-- Build files have been written to: /home/oem/work/casbin-CMake-setup/build

# build makefiles
oem@maxim:~/work/casbin-CMake-setup/build$ make CasbinClientTest
[ 1%] Building CXX object _deps/casbin-build/casbin/CMakeFiles/casbin.dir/cmake_pch.hxx.gch
[ 3%] Building CXX object _deps/casbin-build/casbin/CMakeFiles/casbin.dir/abac_data.cpp.o
[ 5%] Building CXX object _deps/casbin-build/casbin/CMakeFiles/casbin.dir/enforcer.cpp.o
[ 6%] Building CXX object _deps/casbin-build/casbin/CMakeFiles/casbin.dir/enforcer_cached.cpp.o
[ 8%] Building CXX object _deps/casbin-build/casbin/CMakeFiles/casbin.dir/enforcer_synced.cpp.o
[ 10%] Building CXX object _deps/casbin-build/casbin/CMakeFiles/casbin.dir/internal_api.cpp.o
[ 12%] Building CXX object _deps/casbin-build/casbin/CMakeFiles/casbin.dir/logger.cpp.o
[ 13%] Building CXX object _deps/casbin-build/casbin/CMakeFiles/casbin.dir/management_api.cpp.o
[ 15%] Building CXX object _deps/casbin-build/casbin/CMakeFiles/casbin.dir/pch.cpp.o
[ 17%] Building CXX object _deps/casbin-build/casbin/CMakeFiles/casbin.dir/rbac_api.cpp.o
[ 18%] Building CXX object _deps/casbin-build/casbin/CMakeFiles/casbin.dir/rbac_api_with_domains.cpp.o
[ 20%] Building CXX object _deps/casbin-build/casbin/CMakeFiles/casbin.dir/config/config.cpp.o
/home/oem/work/casbin-CMake-setup/build/_deps/casbin-src/casbin/config/config.cpp:17:10: fatal error: pch.h: No such file or directory
17 | #include "pch.h"
| ^~~~~~~
compilation terminated.
make[3]: *** [_deps/casbin-build/casbin/CMakeFiles/casbin.dir/build.make:253: _deps/casbin-build/casbin/CMakeFiles/casbin.dir/config/config.cpp.o] Error 1
make[2]: *** [CMakeFiles/Makefile2:141: _deps/casbin-build/casbin/CMakeFiles/casbin.dir/all] Error 2
make[1]: *** [CMakeFiles/Makefile2:122: CMakeFiles/CasbinClientTest.dir/rule] Error 2
make: *** [Makefile:169: CasbinClientTest] Error 2
```

Can I get help with this issue? What I am doing wrong? Unfortunately I did not find "official" instructions to build.

首先在干净的构建目录中重现报告中的命令：运行 `cmake ..` 和 `make CasbinClientTest`。检查 `_deps/casbin-src/casbin/config/config.cpp` 附近的构建配置以及缺失的 `pch.h`；完成的标准是 target 成功编译，并且项目有清晰的构建说明。

由索引模型根据 Issue 内容生成。