fixed issue with lpthread not being linked properly

[AlbanGauthier]

/usr/bin/ld: libotapputils.a(image_utils.cpp.o): in function cimg_library::cimg::Mutex_info::trylock(unsigned int)': /home/alban/Documents/workspace/otmap/extern/CImg/CImg.h:3068: undefined reference to pthread_mutex_trylock' collect2: error: ld returned 1 exit status make[2]: *** [CMakeFiles/otmap.dir/build.make:86: otmap] Error 1 make[1]: *** [CMakeFiles/Makefile2:148: CMakeFiles/otmap.dir/all] Error 2 make: *** [Makefile:84: all] Error 2