Fix MSVC 2019 build in C++17 mode

Some set commands in CMake files did not work correctly because
quotation is missing. It turns out set(A ${B}) does not set A to
empty if B is empty. set(A "${B}") works also if B is empty.

Author: andreasbuhr

cmake/FindCoroutines.cmake

@@ -152,7 +152,7 @@ if(find_final)
     check_include_file_cxx("coroutine" _CXX_COROUTINES_HAVE_HEADER)
     if(_CXX_COROUTINES_HAVE_HEADER)
         check_cxx_source_compiles("#include <coroutine> \n typedef std::suspend_never blub; \nint main() {} " _CXX_COROUTINES_FINAL_HEADER_COMPILES)
-        set(_CXX_COROUTINES_HAVE_HEADER ${_CXX_COROUTINES_FINAL_HEADER_COMPILES})
+        set(_CXX_COROUTINES_HAVE_HEADER "${_CXX_COROUTINES_FINAL_HEADER_COMPILES}")
     endif()
 
     if(NOT _CXX_COROUTINES_HAVE_HEADER)
@@ -161,9 +161,9 @@ if(find_final)
         check_include_file_cxx("coroutine" _CXX_COROUTINES_HAVE_HEADER_WITH_FLAG)
         if(_CXX_COROUTINES_HAVE_HEADER_WITH_FLAG)
             check_cxx_source_compiles("#include <coroutine> \n typedef std::suspend_never blub; \nint main() {} " _CXX_COROUTINES_FINAL_HEADER_COMPILES_WITH_FLAG)
-            set(_CXX_COROUTINES_HAVE_HEADER_WITH_FLAG ${_CXX_COROUTINES_FINAL_HEADER_COMPILES_WITH_FLAG})
+            set(_CXX_COROUTINES_HAVE_HEADER_WITH_FLAG "${_CXX_COROUTINES_FINAL_HEADER_COMPILES_WITH_FLAG}")
         endif()
-        set(_CXX_COROUTINES_HAVE_HEADER ${_CXX_COROUTINES_HAVE_HEADER_WITH_FLAG})
+        set(_CXX_COROUTINES_HAVE_HEADER "${_CXX_COROUTINES_HAVE_HEADER_WITH_FLAG}")
         cmake_pop_check_state()
     endif()
     mark_as_advanced(_CXX_COROUTINES_HAVE_HEADER)
@@ -182,7 +182,7 @@ if(find_experimental)
         cmake_push_check_state()
         set(CMAKE_REQUIRED_FLAGS "${_CXX_COROUTINES_EXTRA_FLAGS}")
         check_include_file_cxx("experimental/coroutine" _CXX_COROUTINES_HAVE_EXPERIMENTAL_HEADER_WITH_FLAG)
-        set(_CXX_COROUTINES_HAVE_EXPERIMENTAL_HEADER ${_CXX_COROUTINES_HAVE_EXPERIMENTAL_HEADER_WITH_FLAG})
+        set(_CXX_COROUTINES_HAVE_EXPERIMENTAL_HEADER "${_CXX_COROUTINES_HAVE_EXPERIMENTAL_HEADER_WITH_FLAG}")
         cmake_pop_check_state()
     endif()
     mark_as_advanced(_CXX_COROUTINES_HAVE_EXPERIMENTAL_HEADER)
@@ -256,7 +256,7 @@ if(CXX_COROUTINES_HAVE_COROUTINES)
         # Add the -fcoroutines-ts (or /await) flag
         set(CMAKE_REQUIRED_FLAGS "${_CXX_COROUTINES_EXTRA_FLAGS}")
         check_cxx_source_compiles("${code}" CXX_COROUTINES_AWAIT_NEEDED)
-        set(can_link ${CXX_COROUTINES_AWAIT_NEEDED})
+        set(can_link "${CXX_COROUTINES_AWAIT_NEEDED}")
     endif()
 
     if(can_link)