Fixed|Unix|libdeng2: String comparison functions for C code

On Unix platforms, always include <strings.h> for the platform's
string comparison functions when using the libdeng2 C API.

doomsday/libdeng2/include/de/libdeng2.h

@@ -76,6 +76,14 @@
 
 #include <assert.h>
 
+/*
+ * When using the C API, the Qt string functions are not available, so we
+ * must use the platform-specific functions.
+ */
+#if defined(UNIX) && defined(DENG2_C_API_ONLY)
+#  include <strings.h> // strcasecmp etc. 
+#endif
+
 /*
  * The DENG2_PUBLIC macro is used for declaring exported symbols. It must be
  * applied in all exported classes and functions. DEF files are not used for