cmake: disable -Wnon-virtual-dtor when it gives false positives

clang has only been smart enough not to trigger -Wnon-virtual-dtor
warnings on final classes since r208449 (in clang 3.5). Building
with older versions is extremely noisy, so disable the warning
on those compilers.
cmake: actually test -Wcomment

This test was testing nothing, as only -Werror was ever
being added to the compiler flags.

You can see the final nitty-gritty compiler invocation in
CMakeFiles/CMakeOutput.log (for successful tests) and
CMakeFiles/CMakeError.log (for failed tests).

Before:
Building C object CMakeFiles/cmTryCompileExec3385359576.dir/src.c.o
/usr/bin/clang   -fPIC -Wall -W -Wno-unused-parameter -Wwrite-strings -Wmissing-field-initializers -pedantic -Wno-long-long -Wcovered-switch-default  -DC_WCOMMENT_ALLOWS_LINE_WRAP  -Werror   -o CMakeFiles/cmTryCompileExec3385359576.dir/src.c.o   -c /home/nobled/code/llvm-b9/CMakeFiles/CMakeTmp/src.c

After:
Building C object CMakeFiles/cmTryCompileExec3385359576.dir/src.c.o
/usr/bin/clang   -fPIC -Wall -W -Wno-unused-parameter -Wwrite-strings -Wmissing-field-initializers -pedantic -Wno-long-long -Wcovered-switch-default  -DC_WCOMMENT_ALLOWS_LINE_WRAP  -Werror -Wcomment   -o CMakeFiles/cmTryCompileExec3385359576.dir/src.c.o   -c /home/nobled/code/llvm-b9/CMakeFiles/CMakeTmp/src.c