gtk3 gtk.h/gdk.h not buildable

[Jeff Johnson]

The header gtk-3.0/gdk/gdk.h fails if included in a build.
The error is not terribly helpful:

/opt/lsb/include/gtk-3.0/gdk/gdk.h:1547:1: error: expected identifier or
‘(’ before ‘}’ token

the line number is that of the ending brace that's supposed to match the
one from

#ifdef __cplusplus
extern "C" {
#endif

and appears as

#ifdef __cplusplus
}
#endif

If a source file including the referenced header is preprocessed, there does
seem to be a mismatched number of braces, but it's not clear where it comes
from. The header itself has a matched number.