cmake: include directories don't have SYSTEM property which can cause warnings

**Summary**



When integrating the abseil-cpp library into a cmake project, the include directories are not marked as SYSTEM. This enables the project-specific diagnostics to be applied to absl headers.



Is there an good way to mark include directories of libraries as `SYSTEM`? I couldn't find one from researching a bit.



Otherwise, I think it would be useful if abseil had an option to apply `SYSTEM` property to the `target_include_directories`.



**Details**



Here is a minimal cmake project to demonstrate the issue (this should reproduce with most common systems/compilers, I was using Ubuntu 20.04, cmake 3.20, gcc 9.3.0/clang 10.0):



```

matled@fg:~/src/demo% tail -n 10 CMakeLists.txt demo.cc

==> CMakeLists.txt <==

cmake_minimum_required(VERSION 3.5)

project(demo)

add_subdirectory(abseil-cpp)

add_executable(demo demo.cc)

target_link_libraries(demo absl::strings)

target_compile_options(demo PRIVATE -Wconversion -Werror)



==> demo.cc <==

#include <absl/strings/str_cat.h>



int main() {

  printf("%s\n", absl::StrCat("foo", "bar").c_str());

  return 0;

}

matled@fg:~/src/demo% rm -rf build && cmake -S . -B build && cmake --build build --target demo --verbose

-- The C compiler identification is GNU 9.3.0

-- The CXX compiler identification is GNU 9.3.0

[...]

/usr/bin/c++  -I/home/matled/src/demo/abseil-cpp -Wconversion -Werror [...]

In file included from /home/matled/src/demo/demo.cc:1:

/home/matled/src/demo/abseil-cpp/absl/strings/str_cat.h: In constructor ‘absl::Hex::Hex(absl::PadSpec, uint64_t)’:

/home/matled/src/demo/abseil-cpp/absl/strings/str_cat.h:171:19: error: conversion from ‘int’ to ‘uint8_t’ {aka ‘unsigned char’} may change value [-Werror=conversion]

[...]

cc1plus: all warnings being treated as errors

make[3]: *** [CMakeFiles/demo.dir/build.make:76: CMakeFiles/demo.dir/demo.cc.o] Error 1

make[3]: Leaving directory '/mnt/c/Users/matled/source/demo/build'

make[2]: *** [CMakeFiles/Makefile2:551: CMakeFiles/demo.dir/all] Error 2

make[2]: Leaving directory '/mnt/c/Users/matled/source/demo/build'

make[1]: *** [CMakeFiles/Makefile2:558: CMakeFiles/demo.dir/rule] Error 2

make[1]: Leaving directory '/mnt/c/Users/matled/source/demo/build'

make: *** [Makefile:124: demo] Error 2

```



When include directories use the `-isystem` flag instead of `-I`, warnings are suppressed. cmake sets this option based on the `SYSTEM` property being specified on `target_include_directories`. I could not find an easy way to set this option outside of the abseil cmake files.



I've verified manually that adding the `SYSTEM` property to the two uses of `target_include_directories` in `function(absl_cc_library)` solves the issue.



An option in the abseil-cpp cmake file to mark the include directories as `SYSTEM` would be one solution for this.

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