specify lm for weak linkers that can't find libmath fixes #7

When building on Docker, I got linker errors around libmath.  This fixes my issue.  The way libsass is imported from a parent directory makes showing my docker example impractical.  This can be seen in my project which was based off your work.  Thanks btw!

https://github.com/drewwells/sprite_sass/commit/5c05b3

gofmt added some whitespace changes