Reuse compiled objects to build the libraries

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>

src/CMakeLists.txt

@@ -57,9 +57,13 @@ ELSEIF (UNIX)
     ENDIF()
 ENDIF()
 
+## common compilation for libpaho-mqtt3c and libpaho-mqtt3a
+ADD_LIBRARY(common_obj OBJECT ${common_src})
+SET_PROPERTY(TARGET common_obj PROPERTY POSITION_INDEPENDENT_CODE ON)
+
 ADD_EXECUTABLE(MQTTVersion MQTTVersion.c)
-ADD_LIBRARY(paho-mqtt3c SHARED ${common_src} MQTTClient.c)
-ADD_LIBRARY(paho-mqtt3a SHARED ${common_src} MQTTAsync.c)
+ADD_LIBRARY(paho-mqtt3c SHARED $<TARGET_OBJECTS:common_obj> MQTTClient.c)
+ADD_LIBRARY(paho-mqtt3a SHARED $<TARGET_OBJECTS:common_obj> MQTTAsync.c)
 TARGET_LINK_LIBRARIES(paho-mqtt3c ${LIBS_SYSTEM})
 TARGET_LINK_LIBRARIES(paho-mqtt3a ${LIBS_SYSTEM})
 TARGET_LINK_LIBRARIES(MQTTVersion paho-mqtt3a paho-mqtt3c ${LIBS_SYSTEM})
@@ -85,8 +89,12 @@ IF (PAHO_WITH_SSL)
     INCLUDE_DIRECTORIES(
         ${OPENSSL_INC_SEARCH_PATH}
         )
-    ADD_LIBRARY(paho-mqtt3cs SHARED ${common_src} MQTTClient.c SSLSocket.c)
-    ADD_LIBRARY(paho-mqtt3as SHARED ${common_src} MQTTAsync.c SSLSocket.c)
+    ## common compilation for libpaho-mqtt3cs and libpaho-mqtt3as
+    ## Note: SSL libraries must be recompiled due ifdefs
+    ADD_LIBRARY(common_ssl_obj OBJECT ${common_src})
+    SET_PROPERTY(TARGET common_ssl_obj PROPERTY POSITION_INDEPENDENT_CODE ON)
+    ADD_LIBRARY(paho-mqtt3cs SHARED $<TARGET_OBJECTS:common_ssl_obj> MQTTClient.c SSLSocket.c)
+    ADD_LIBRARY(paho-mqtt3as SHARED $<TARGET_OBJECTS:common_ssl_obj> MQTTAsync.c SSLSocket.c)
     TARGET_LINK_LIBRARIES(paho-mqtt3cs ${OPENSSL_LIBRARIES} ${LIBS_SYSTEM})
     TARGET_LINK_LIBRARIES(paho-mqtt3as ${OPENSSL_LIBRARIES} ${LIBS_SYSTEM})
     SET_TARGET_PROPERTIES(