Add additional handling for AddressSanitizer

[aceckel]

This commit is a continuation of commit 2230e3e ("ignore sanitizer"),
which added the "no_sanitize_address" attribute to ctest_main to
prevent ASan from flagging reads while discovering the registered test
structs. Clang's ASan implementation adds a redzone around globals, so
some additional handling is required.

Starting with clang 4.0, the "no_sanitize_address" can also be applied
to globals to inhibit creating the globals redzone. However, for
earlier versions of clang (3.1 was the first to include ASan), we need
to work around the redzone. Thanks to ASAN_UNPOISON_MEMORY_REGION(),
this is possible. By unpoisoning the memory region and skipping over
any alignment padding, we are able to derive the test list, but the
"magic" value is now required to bookend the ctest struct.

This commit also attempts to make the compiler platform and version
checking logic more readable.

[aceckel]

An alternative to trying to work around sanitizers would be to change the test discovery method to not involve the use of a linker set. Linker sets are a handy tool, but are difficult to do portably (as this PR emphasizes). An approach that shouldn't anger sanitizers would be to construct a global linked list of tests using static constructors (mentioned as a solution to a different issue in #35 (comment)). I think we should consider this approach since it may kill two birds with one stone (fixes both the sanitizer issue and the Windows incremental linking issue while adding comparatively little platform-specific code).