[PATCH] build: support building static library

[ComixHe]

According to your suggestions:

 > That is also fine with me, anyway, liberofsfuse is a very special
 > case for us, I'd like to just build a static library for
 > erofsfuse only (because liberofs will be exported as a dynamic
 > library in later versions, I don't want to rely on random libfuse
 > interface).
 >
 > For other usage, I'd suggest form some formal APIs in liberofs
 > instead.

I create a new patch which only for exporting 'liberofsfuse.a'.

Thanks,

He YuMing