Add support for pgettext()

Primarily gives it a shorthand, but also make gcc tolerate its
use for format strings.

po/CMakeLists.txt

@@ -23,6 +23,7 @@ if (GETTEXT_XGETTEXT_EXECUTABLE)
       "--output=${CMAKE_CURRENT_SOURCE_DIR}/tigervnc.pot"
       --default-domain=tigervnc
       --keyword=_
+      --keyword=p_:1c,2
       --keyword=N_
       "--copyright-holder=TigerVNC Team and many others \(see README.txt\)"
       --msgid-bugs-address=tigervnc-devel@googlegroups.com

vncviewer/i18n.h

@@ -22,7 +22,16 @@
 
 #include "gettext.h"
 
+/* Need to tell gcc that pgettext() doesn't screw up format strings */
+#ifdef __GNUC__
+static const char *
+pgettext_aux (const char *domain,
+              const char *msg_ctxt_id, const char *msgid,
+              int category) __attribute__ ((format_arg (3)));
+#endif
+
 #define _(String) gettext (String)
+#define p_(Context, String) pgettext (Context, String)
 #define N_(String) gettext_noop (String)
 
 #endif /* _I18N_H */