core: add CMake option "ENABLE_CODE_COVERAGE" to compile with code coverage options

This option is OFF by default and should be enabled only for tests, to measure
test coverage.

src/plugins/exec/CMakeLists.txt

@@ -25,6 +25,6 @@ exec-completion.c exec-completion.h
 exec-config.c exec-config.h)
 set_target_properties(exec PROPERTIES PREFIX "")
 
-target_link_libraries(exec)
+target_link_libraries(exec coverage_config)
 
 install(TARGETS exec LIBRARY DESTINATION ${LIBDIR}/plugins)