POSIX-compliant sed regex in complexity measurement instrumentation script

The shell script scripts/prepare_instrumentation.sh performing the instrumentation for the complexity measurement uses sed with regular expressions for modifying lib_com/options.h. The regex uses \s for whitespace. This works in GNU sed, but the sed in Mac is limited to POSIX definitions. This means that [[:space:]] should be used instead. This is supported also by GNU sed.