ldflags: link against libm

[btwotch]

otherwise I get:

# github.com/mitchellh/go-z3
/usr/bin/ld: ./libz3.a(hwf.o): undefined reference to symbol 'remainder@@GLIBC_2.2.5'
/usr/bin/ld: /usr/lib64/libm.so.6: error adding symbols: DSO missing from command line
collect2: error: ld returned 1 exit status
make: *** [Makefile:25: test] Error 2