Merge static library

We not only can add source code files and also add some object(.o)/library(.a) files to the target directly for the api: add_files

It is different with another interface: add_links

• add_links：It only links library, .e.g -lxxxx and it's target is only binary program and share library.
• add_files：It will merge the static library into the target.

####Example

target("test")
    
     -- create static library: libtest.a
     set_kind("static")

     -- add source files to the libtest.a
     add_files("src/*.c")

     -- add object files to the libtest.a
     add_files("obj/*.o")

     -- merge static librarys to the libtest.a
     add_files("lib/*.a")

And the target kind also can be binary or shared

target("test2")
    
     -- create a shared library: libtest2.so
     set_kind("shared")

     -- add object files to the libtest2.so
     add_files("obj/*.o")

     -- merge libtest.a to the libtest2.so, but not link
     add_files("lib/libtest.a")

     -- add some source files
     add_files("src/*.c")