silent-rules tests: fix spurious failures with Sun Studio C++ * tests/silentcxx.test: The C++ compiler from Sun Studio is named `CC'. Account for this in our grepping checks on the make output. Since we are at it, throw in a couple of improvements to comments and formatting. * tests/silent-many-generic.test: The C++ compiler from Sun Studio is named `CC', and this can cause spurious failures in our grepping of the make output. Work around this by using a wrapper script around the C++ compiler (generated on the fly), since filtering the make output proved to be too fragile.