BUILD: makefile: adjust the sed expression of "make help" for solaris

BUILD: makefile: adjust the sed expression of "make help" for solaris

Solaris's sed doesn't take the 'p' argument on the 's' command so
nothing is printed. Just passing ';p' fixes this without affecting
other implementations. Additionally, optional characters cannot be
matched using a question mark, which is always searched verbatim, so
the leading '#' wasn't stripped. Using \{0,1\} works fine everywhere
so let's use this instead.