[CMake] merge static libraries with ar

[Timur Ivanov]

Hello again :)

Suppose i have

ADD_SUBDIRECTORY(foo)  # produce libfoo.a
ADD_SUBDIRECTORY(bar)  # produce libbar.a
ADD_SUBDIRECTORY(baz) # produce libbaz.a

Now i need to merge all of them into single library libcool.a. Would
you like to give me some hints for that task ?