depcomp: fix bugs in tests and in the depcomp script * lib/depcomp (gcc): Remove duplicated `-e' from sed invocation. * tests/depcomp10.test: Make it executable. Fix a blunder that has left part of an intended comment not prefixed by `#', thus causing shell syntax errors. In this same comment, break a too-long reported error message on multiple lines, for clarity. Add reference to the relevant bug report. Add a comment which explains why the test result 'skipped' if the first "make" call fails. Add other useful comments. * tests/depcomp9.test: Slightly improve comments.