Fix build for mingw and dll linking

[mgreter]

Made windows mingw compliation working:
https://ci.appveyor.com/project/mgreter/libsass/build/1.0.25
In preparation of https://github.com/mgreter/libsass/tree/feature/build-dll-win-mingw

[rototor]

This just broke the build on Mac OS X .... (10.10 if that matters)

cc -static -Wall -O2 -std=c++0x -stdlib=libc++ -fPIC -o bin/sassc sassc.o ../libsass//lib/libsass.a -lstdc++ -lm
ld: library not found for -lcrt0.o

==> You can not build a real static binary on Darwin... I´ll open a pullrequest with the fix.

[mgreter]

Hmm, IMO the only difference should be -static, but I think I needed that to make it compile on mingw. But I'll wait for your PR! Maybe you would be willing to write down how you compile libsass on mac os x? I cannot test that and I'm currently updating the docs in regard of building on linux and windows.