use ADD_CONVENIENCE_LIBRARY when building libservices, because

it is a static library that links with shared libraries, so strictly speaking it should
have -fPIC or equivalent flags. Also, it must always build as static no matter
whether  BUILD_SHARED_LIBS is set.

libservices/CMakeLists.txt

@@ -22,5 +22,5 @@ SET(MYSQLSERVICES_SOURCES
   my_thread_scheduler_service.c
   progress_report_service.c)
 
-ADD_LIBRARY(mysqlservices ${MYSQLSERVICES_SOURCES})
+ADD_CONVENIENCE_LIBRARY(mysqlservices ${MYSQLSERVICES_SOURCES})
 INSTALL(TARGETS mysqlservices DESTINATION ${INSTALL_LIBDIR} COMPONENT Development)