Added --delete to gcovr second invocation

This allows to perform a correct diagnostic when sources/tests evolve
for non clean build tree

CMakeLists.txt

@@ -180,7 +180,7 @@ if("${CMAKE_BUILD_TYPE}" STREQUAL "Coverage")
     ALL
     COMMAND ${CMAKE_COMMAND} -E remove_directory "${PASTIS_BINARY_DIR}/coverage"
     COMMAND ${CMAKE_COMMAND} -E make_directory "${PASTIS_BINARY_DIR}/coverage"
-    COMMAND ${GCOVR} ${GCOVR_OPTIONS} --html --html-details -o "${PASTIS_BINARY_DIR}/coverage/coverage.html"
+    COMMAND ${GCOVR} ${GCOVR_OPTIONS} --delete --html --html-details -o "${PASTIS_BINARY_DIR}/coverage/coverage.html"
     DEPENDS coverage
     COMMENT "Building coverage html report."
     WORKING_DIRECTORY "${PASTIS_BINARY_DIR}"