Add #ifndefs around sdiv/smul/sadd/ssub in the C++ interface.
Add #ifndefs around sdiv/smul/sadd/ssub in the C++ interface.

Closes issue #19.



git-svn-id: 1a3934ecdee2dfe077e3a1b34320424617f14597@87 d3e1167c-abe1-51d5-8199-f9061ebe54e4

ijacquez/libfixmath