Do not generate and use

A C++ program can also use usage_exit.c.

Change-Id: I16234cee84a7269f0c5a19f03efb54884eba80a1
diff --git a/CMakeLists.txt b/CMakeLists.txt
index f4c0571..6243ec7 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -368,16 +368,13 @@
-# Generate C/C++ files containing the function usage_exit(). Users of the
+# Generate a C file containing the function usage_exit(). Users of the
 # aom_common_app_util library must define this function. This is a convenience
 # to allow omission of the function from applications that might want to use
 # other pieces of the util support without defining usage_exit().
 file(WRITE "${AOM_GEN_SRC_DIR}/usage_exit.c"
      "#include <stdlib.h>\n\n#include \"common/tools_common.h\"\n\n"
      "void usage_exit(void) { exit(EXIT_FAILURE); }\n")
-     "#include <stdlib.h>\n\n#include \"common/tools_common.h\"\n\n"
-     "extern \"C\" void usage_exit(void) { exit(EXIT_FAILURE); }\n")
 # Application and application support targets.
@@ -674,7 +671,7 @@
-    add_executable(dump_obu "${AOM_GEN_SRC_DIR}/"
+    add_executable(dump_obu "${AOM_GEN_SRC_DIR}/usage_exit.c"