CMake: introduce a coverage target

to produce coverage information while running the tests, call cmake with
the following options:

cmake -DCMAKE_C_FLAGS=-coverage -DCMAKE_CXX_FLAGS=-coverage <build-dir>

Signed-off-by: Kai Buschulte <kai.buschulte@cedalo.com>

test/CMakeLists.txt

@@ -2,3 +2,10 @@ add_subdirectory(broker)
 add_subdirectory(client)
 add_subdirectory(lib)
 add_subdirectory(unit)
+
+add_custom_target(coverage
+    COMMAND lcov --quiet --capture --directory . --output-file ${PROJECT_BINARY_DIR}/coverage.info --no-external
+    COMMAND genhtml --quiet ${PROJECT_BINARY_DIR}/coverage.info --output-directory ${PROJECT_BINARY_DIR}/coverage
+    COMMENT "Generating coverage.info and coverage/index.html in ${PROJECT_BINARY_DIR}"
+    WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}
+)