add '-lm'

I got following errors on Linux(GCC).
`libm` must be linked because `src/number.c` uses some math library, `cos`, `sin` etc.

```
gcc -Wall -O3 -g -DDEBUG=1 tools/main.c -o bin/picrin -I./include -L./lib -lpicrin -lreadline
./lib/libpicrin.so: undefined reference to `lround'
./lib/libpicrin.so: undefined reference to `atan2'
./lib/libpicrin.so: undefined reference to `acos'
./lib/libpicrin.so: undefined reference to `sin'
./lib/libpicrin.so: undefined reference to `atan'
./lib/libpicrin.so: undefined reference to `asin'
./lib/libpicrin.so: undefined reference to `exp'
./lib/libpicrin.so: undefined reference to `trunc'
./lib/libpicrin.so: undefined reference to `tan'
./lib/libpicrin.so: undefined reference to `cos'
./lib/libpicrin.so: undefined reference to `log'
./lib/libpicrin.so: undefined reference to `pow'
```