[flang] Move `external-hello-world` to flang/examples

[awarzynski]

Summary

As external-hello-world is not really a test, I am moving it from
flang/unittest/Runtime to flang/examples. IMHO it makes much more
sense as an example. I've not modified the source code (apart from
adjusting the include paths).

flang/CMakeLists.txt

 add_subdirectory(cmake/modules)
 
 option(FLANG_BUILD_TOOLS
   "Build the Flang tools. If OFF, just generate build targets." ON)
 if (FLANG_BUILD_TOOLS)
   add_subdirectory(tools)
 endif()
 add_subdirectory(runtime)
+add_subdirectory(examples)
 
 if (FLANG_INCLUDE_TESTS)
   add_subdirectory(test)
   if (FLANG_GTEST_AVAIL)
     add_subdirectory(unittests)
   endif ()
 endif()
 

flang/examples/CMakeLists.txt

+# This test is not run by default as it requires input.
+add_executable(external-hello-world
+  external-hello.cpp
+)
+
+target_link_libraries(external-hello-world
+  FortranRuntime
+)

flang/examples/external-hello.cpp

-#include "../../runtime/io-api.h"
-#include "../../runtime/main.h"
-#include "../../runtime/stop.h"
+#include "../runtime/io-api.h"
+#include "../runtime/main.h"
+#include "../runtime/stop.h"
 #include <cstring>
 #include <limits>
 
 using namespace Fortran::runtime::io;
 
 void output1() {
   auto io{IONAME(BeginExternalListOutput)()};
   const char str[]{"Hello, world!"};

flang/unittests/Runtime/CMakeLists.txt

 else()
   llvm_map_components_to_libnames(llvm_libs Support)
 endif()
 target_link_libraries(RuntimeTesting
   FortranRuntime
   ${llvm_libs}
 )
 
-# This test is not run by default as it requires input.
-add_executable(external-hello-world
-  external-hello.cpp
-)
-
-target_link_libraries(external-hello-world
-  FortranRuntime
-)
-
 add_flang_nongtest_unittest(external-io
   RuntimeTesting
   FortranRuntime
 )