client/common: create a combined .a on mac

Generate a combined archive of all the client related functions (channels/common..) to
minimize dependencies for linking.
The archive is called libfreerdp-client-combined.a and is only generated on iOS when doing a
monolithic static build.
client/common: also include libfreerdp-client in combined archive
ios: link against libfreerdp-client-combined.a
Merge pull request #1129 from bmiklautz/osx_static_build

client/common: create a combined .a on mac