Added libm to makefile to stop error on new systems as follows

gcc  -I../include `pkg-config --cflags opencv gtk+-2.0` siftfeat.c -o ../bin/siftfeat -L../lib -lopensift `pkg-config --libs opencv gtk+-2.0`
/usr/bin/ld: ../lib/libopensift.a(sift.o): undefined reference to symbol 'exp@@GLIBC_2.2.5'
/usr/bin/ld: note: 'exp@@GLIBC_2.2.5' is defined in DSO /lib64/libm.so.6 so try adding it to the linker command line
/lib64/libm.so.6: could not read symbols: Invalid operation
collect2: error: ld returned 1 exit status