Generalize CMake flag handling to support GCC

While clang uses the -fcoroutines-ts flag to add support for the
Coroutines TS, GCC uses the -fcoroutines flag.

This commit refactors the cmake finder to support both.

cmake/FindCppcoroCoroutines.cmake

@@ -3,19 +3,24 @@ include(CheckIncludeFileCXX)
 include(FindPackageHandleStandardArgs)
 
 check_cxx_compiler_flag(/await Coroutines_SUPPORTS_MS_FLAG)
-check_cxx_compiler_flag(-fcoroutines-ts Coroutines_SUPPORTS_GNU_FLAG)
-if(Coroutines_SUPPORTS_MS_FLAG OR Coroutines_SUPPORTS_GNU_FLAG)
+check_cxx_compiler_flag(-fcoroutines-ts Coroutines_SUPPORTS_CLANG_FLAG)
+check_cxx_compiler_flag(-fcoroutines Coroutines_SUPPORTS_GCC_FLAG)
+if(Coroutines_SUPPORTS_MS_FLAG OR Coroutines_SUPPORTS_CLANG_FLAG OR Coroutines_SUPPORTS_GCC_FLAG)
     set(Coroutines_COMPILER_SUPPORT ON)
 endif()
 
+set(Coroutines_ADDITIONAL_FLAG "")
 if(Coroutines_SUPPORTS_MS_FLAG)
-    check_include_file_cxx("coroutine" Coroutines_STANDARD_LIBRARY_SUPPORT "/await")
-    check_include_file_cxx("experimental/coroutine" Coroutines_EXPERIMENTAL_LIBRARY_SUPPORT "/await")
-elseif(Coroutines_SUPPORTS_GNU_FLAG)
-    check_include_file_cxx("coroutine" Coroutines_STANDARD_LIBRARY_SUPPORT "-fcoroutines-ts")
-    check_include_file_cxx("experimental/coroutine" Coroutines_EXPERIMENTAL_LIBRARY_SUPPORT "-fcoroutines-ts")
+    set(Coroutines_ADDITIONAL_FLAG "/await")
+elseif(Coroutines_SUPPORTS_CLANG_FLAG)
+    set(Coroutines_ADDITIONAL_FLAG "-fcoroutines-ts")
+elseif(Coroutines_SUPPORTS_GCC_FLAG)
+    set(Coroutines_ADDITIONAL_FLAG "-fcoroutines")
 endif()
 
+check_include_file_cxx("coroutine" Coroutines_STANDARD_LIBRARY_SUPPORT ${Coroutines_ADDITIONAL_FLAG})
+check_include_file_cxx("experimental/coroutine" Coroutines_EXPERIMENTAL_LIBRARY_SUPPORT ${Coroutines_ADDITIONAL_FLAG})
+
 if(Coroutines_EXPERIMENTAL_LIBRARY_SUPPORT OR Coroutines_STANDARD_LIBRARY_SUPPORT)
     set(Coroutines_LIBRARY_SUPPORT ON)
 endif()
@@ -29,8 +34,4 @@ if(NOT CppcoroCoroutines_FOUND OR TARGET cppcoro::coroutines)
 endif()
 
 add_library(cppcoro::coroutines INTERFACE IMPORTED)
-if(Coroutines_SUPPORTS_MS_FLAG)
-    target_compile_options(cppcoro::coroutines INTERFACE /await)
-elseif(Coroutines_SUPPORTS_GNU_FLAG)
-    target_compile_options(cppcoro::coroutines INTERFACE -fcoroutines-ts)
-endif()
+target_compile_options(cppcoro::coroutines INTERFACE ${Coroutines_ADDITIONAL_FLAG})