linking to libopensift from C++

[sprock]

Hello,
I'm attempting to call libopensift from C++ and running into linker
errors:

clang++ -g -I/opt/OpenSIFT/include -I/usr/local/include/opencv
-I/usr/local/include -I/usr/local/include -I/usr/local/include/glib-2.0
-I/usr/local/lib/glib-2.0/include -I/usr/local/include/libxml2
-I/usr/local/include/libgsf-1 -I/usr/local/include/GraphicsMagick
-I/usr/local/include/orc-0.4 -I/usr/local/include/OpenEXR
-I/usr/local/include/poppler/glib -I/usr/local/include/cairo
-I/usr/local/include/pixman-1 -I/usr/local/include/freetype2
-I/usr/local/include/libdrm -pthread -D_THREAD_SAFE
-I/usr/local/include/libpng16 -I/usr/local/include/poppler
-I/usr/local/include/librsvg-2.0 -I/usr/local/include/gdk-pixbuf-2.0
-pthread -I/usr/local/include/pango-1.0 -I/usr/local/include/harfbuzz
-c Mosaic.cxx -o mosaic.o

clang++ -L/usr/local/lib -lvips-cpp -lvips -lgobject-2.0 -lglib-2.0
-lintl -L/usr/local/lib -lopencv_calib3d -lopencv_contrib -lopencv_core
-lopencv_features2d -lopencv_flann -lopencv_gpu -lopencv_highgui
-lopencv_imgproc -lopencv_legacy -lopencv_ml -lopencv_nonfree
-lopencv_objdetect -lopencv_ocl -lopencv_photo -lopencv_stitching
-lopencv_superres -lopencv_ts -lopencv_video -lopencv_videostab -ltbb
-lpthread -lm -L/opt/OpenSIFT/lib -lopensift -o mosaic mosaic.o

mosaic.o: In function dem(std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >, std::__1::basic_string<char, std::__1::char_traits<char>, std::__1::allocator<char> >)': /home/rmason/Programming/C/VipsMosaic2/Mosaic.cxx:383: undefined reference to sift_features(_IplImage*, feature**)'
/home/rmason/Programming/C/VipsMosaic2/Mosaic.cxx:384: undefined reference to sift_features(_IplImage*, feature**)' /home/rmason/Programming/C/VipsMosaic2/Mosaic.cxx:389: undefined reference to kdtree_bbf_knn(kd_node*, feature*, int, feature***, int)'
/home/rmason/Programming/C/VipsMosaic2/Mosaic.cxx:392: undefined reference to descr_dist_sq(feature*, feature*)' /home/rmason/Programming/C/VipsMosaic2/Mosaic.cxx:393: undefined reference to descr_dist_sq(feature*, feature*)'
clang++: error: linker command failed with exit code 1 (use -v to see invocation)

I've tried various versions of the link line, modified sift.h to declare
sift_features extern "C", but to no avail.

If you have suggestions on how to link C++ against libsift I would be
most grateful to hear them.

Thanks,
Roger
FreeBSD 10.3