Fix libm missing link error by adding -lm

Linking in Ubuntu 13.10, gcc version 4.8.1:

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(imgfeatures.o): undefined reference to symbol 'atan2@@GLIBC_2.0'
/lib/i386-linux-gnu/libm.so.6: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status

Adding -lm to the Makefile can fix the problem.