make sure fast string operator+ is used only by LO code

Change-Id: Iaa8fc379e4d032931c0f60a3e3525783d8d28964

sal/inc/rtl/stringutils.hxx

@@ -35,11 +35,13 @@
 // (e.g. for debugging).
 #ifndef RTL_DISABLE_FAST_STRING
 #ifndef HAVE_SFINAE_ANONYMOUS_BROKEN
-// Enable fast string concatenation.
 // This feature is not part of public API and is meant to be used only internally by LibreOffice.
+#ifdef LIBO_INTERNAL_ONLY
+// Enable fast string concatenation.
 #define RTL_FAST_STRING
 #endif
 #endif
+#endif
 
 // The unittest uses slightly different code to help check that the proper
 // calls are made. The class is put into a different namespace to make

solenv/gbuild/gbuild.mk

@@ -197,9 +197,10 @@ gb_GLOBALDEFS := \
 	$(gb_COMPILERDEFS) \
 	$(gb_CPUDEFS) \
 
-# This is used to tag deprecation for API we want to
+# This is used to detect whether LibreOffice is being built (as opposed to building
+# 3rd-party code). Used for tag deprecation for API we want to
 # ensure is not used at all externally while we clean
-# out our internal usage.
+# out our internal usage, for code in sal/ that should be used only internally, etc.
 gb_GLOBALDEFS += \
 	-DLIBO_INTERNAL_ONLY \