Update nasa#940, add ifndef values for new fields

Adds an #ifndef empty string for the two new fields, which
at the very least prevents IDEs (e.g. Eclipse) from redlining
the value as an undefined symbol.

(It is always passed in on command line when building via CMake)

cmake/target/src/target_config.c

@@ -54,6 +54,13 @@
 #define CFE_SPACECRAFT_ID_VALUE     0x42
 #endif
 
+#ifndef CFE_DEFAULT_MODULE_EXTENSION
+#define CFE_DEFAULT_MODULE_EXTENSION    ""
+#endif
+
+#ifndef CFE_DEFAULT_CORE_FILENAME
+#define CFE_DEFAULT_CORE_FILENAME       ""
+#endif
 
 Target_CfeConfigData GLOBAL_CFE_CONFIGDATA =
 {