Fix ptr-comparison-to-string-literal error

A new error raised in the latest versions of gcc and clang.

The address of string literals is undefined in the C standard so the
compiler raises an error if one tries to test for string equality by
comparing them. A better fix would be to replace QOF_ID strings with an
int-based identifier; an even better one would get rid of QOF_ID
entirely and use the C++ type system.

libgnucash/engine/qofid.h

@@ -96,27 +96,7 @@ typedef struct QofCollection_s QofCollection;
 #define QOF_ID_BOOK           "Book"
 #define QOF_ID_SESSION        "Session"
 
-/** Inline string comparison; compiler will optimize away most of this */
-#ifndef _MSC_VER
-# define QSTRCMP(da,db) ({                \
-  gint val = 0;                          \
-  if ((da) && (db)) {                    \
-    if ((da) != (db)) {                  \
-      val = strcmp ((da), (db));         \
-    }                                    \
-  } else                                 \
-  if ((!(da)) && (db)) {                 \
-    val = -1;                            \
-  } else                                 \
-  if ((da) && (!(db))) {                 \
-    val = 1;                             \
-  }                                      \
-  val; /* block assumes value of last statement */  \
-})
-#else
-/* MSVC: Simply use g_strcmp */
-# define QSTRCMP g_strcmp0
-#endif
+#define QSTRCMP g_strcmp0
 
 /** return TRUE if object is of the given type */
 #define QOF_CHECK_TYPE(obj,type) (((obj) != NULL) && \