cmake: Fix potential file name collision in Xcode.

When a target has no C++ sources, but includes C++ object files
Xcode requires special treatment. A dummy source file is created
using the target's name to force the CMake Xcode generator to use
C++ as link language.

Append "_dummy" to the dummy source file's basename to avoid
collisions with the target's main source file when it
is target.cc.

Also adds a TODO to avoid adding the dummy source file when the
target being adjusted already has C++ sources.

Change-Id: I9795f6490e88eae6afc3a5901e6ee76a36cc801a

CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5cf320a..74edf8e 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -498,6 +498,8 @@
 endif ()
 
 if (XCODE)
+  # TODO(tomfinegan): Make sure target has no C++ files before doing this as
+  # it's not necessary in that case.
   if (CONFIG_LIBYUV OR CONFIG_WEBM_IO)
     # The Xcode generator does not obey LINKER_LANGUAGE. Because of the issue
     # what looks like a C++ file needs to be in any target that Xcode will link

build/cmake/util.cmake

diff --git a/build/cmake/util.cmake b/build/cmake/util.cmake
index d6c4322..bbb6992 100644
--- a/build/cmake/util.cmake
+++ b/build/cmake/util.cmake
@@ -15,7 +15,7 @@
 # returns the full path to the dummy source file via the $out_file_path
 # parameter.
 function (create_dummy_source_file basename extension out_file_path)
-  set(dummy_source_file "${AOM_CONFIG_DIR}/${basename}.${extension}")
+  set(dummy_source_file "${AOM_CONFIG_DIR}/${basename}_dummy.${extension}")
   file(WRITE "${dummy_source_file}"
        "// Generated file. DO NOT EDIT!\n"
        "// ${target_name} needs a ${extension} file to force link language, \n"