Expose C_ function (gettext with context string) to guile code

First use is for the document link short code (L)

bindings/core-utils.i

@@ -28,6 +28,7 @@
 #include <gnc-filepath-utils.h>
 #include <gnc-locale-utils.h>
 #include <glib.h>
+#include <glib/gi18n.h>
 #include <gnc-version.h>
 #include <libintl.h>
 
@@ -158,6 +159,20 @@ gchar *gnc_locale_name (void);
 
 %rename ("gnc:gettext") gettext;
 extern const char* gettext(const char*);
+%rename ("gnc:C_gettext") wrap_C_;
+%inline %{
+    /* This helper function wraps the C_() macro in to a function.
+       Direct wrapping results in a compiler error on direct string concatenation
+       inside the macro expansion, so I'm making a detour via g_strconcat */
+    const char* wrap_C_(const char* context, const char* msg);
+    const char* wrap_C_(const char* context, const char* msg)
+    {
+        gchar* combo = g_strconcat (context, "\004", msg, NULL);
+        const gchar* translated = g_dpgettext (NULL, combo, strlen (context) + 1);
+        g_free (combo);
+        return translated;
+    }
+%}
 %rename ("gnc-utf8?") wrap_gnc_utf8_validate;
 %inline %{
   /* This helper function wraps gnc_utf8_validate() into a predicate. */

bindings/guile/core-utils.scm

@@ -27,6 +27,7 @@
 (define-module (gnucash core-utils)
   #:export (N_
             G_
+            C_
             gnc:version))
 
 ;; Guile 2 needs to find the symbols from the extension at compile time already
@@ -42,6 +43,7 @@
 
 ;; gettext functions
 (define G_ gnc:gettext)
+(define C_ gnc:C-gettext)
 (define-syntax-rule (N_ x) x)
 
 ;; the following will define _ to call gnc:gettext for guile up to

gnucash/report/trep-engine.scm

@@ -1255,8 +1255,8 @@ be excluded from periodic reporting.")
                                              (gnc:html-transaction-doclink-anchor
                                               (xaccSplitGetParent split)
                                               ;; Translators: 'L' is short for Linked Document
-                                              (G_ "L"))
+                                              (C_ "Column header for 'Document Link'" "L"))
-                                             (G_ "L"))))))))
+                                             (C_ "Column header for 'Document Link'" "L"))))))))
 
                (add-if (column-uses? 'price)
                        (vector (G_ "Price")