Implement --enable-sanitizers[=sanitizers]

Similar to debugging with Valgrind, it's very useful to enable
-fsanitize= switches to catch bugs at runtime.

$ ./configure
Result: no sanitizer enabled

$ ./configure --enable-sanitizers
Result: -fsanitize=address,undefined

$ ./configure --enable-sanitizers=address,thread,undefined
Result: -fsanitize=address,thread,undefined

$ ./configure --enable-sanitizers=undefined
Result: -fsanitize=undefined
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