Actually try to use EXIF support if it's enabled.

--- inkscape-0.91pre3/src/extension/internal/image-resolution.cpp
+++ inkscape-0.91pre3/src/extension/internal/image-resolution.cpp
@@ -18,6 +18,7 @@
 #include <png.h>
 
 #ifdef HAVE_EXIF
+#define IR_TRY_EXIF 1
 #include <math.h>
 #include <libexif/exif-data.h>
 #endif