Use the CMake feature called Object Library. It decreases compilation
by 30%-50%. It avoids compiling the sources more than once.

But one cannot use this feature if there is conditional compilation
flags. For example, it's required to compile the SSL code again
because of the OPENSSL ifdefs.
Signed-off-by: Guilherme Maciel Ferreira <guilherme.maciel.ferreira@gmail.com>