EnableCoverageReport macro does not append to target properties correctly

[James Henstridge]

In the EnableCoverageReport macro, the properties of various targets are altered with commands like:

    SET_PROPERTY(TARGET ${T} APPEND_STRING PROPERTY COMPILE_FLAGS "-g --coverage ")

If the target has its COMPILE_FLAGS set to e.g. "-foo", then this results in them being set to "-foo-g --coverage ", which results in a compile failure. If we are appending to the property, then there should be a space at the beginning of what we're appending. The space at the end seems superfluous though.