Create OBJECT libraries for all "convenience" libraries.
Merge object libraries using the cmake TARGET_SOURCES construct.

One exception for Windows: add extra linker flags for link.exe in
order to merge OpenSSL libs into mysqlclient

One exception for MacOS: merging of OBJECT libraries does not work with Xcode,
so we use libtool directly to merge STATIC libraries.

This patch raises cmake minimum required version to 3.5.1

Change-Id: I3ece5e1c2c9f6402dc620ae798716c72a09f57e0