fix error: #pragma GCC diagnostic not allowed inside functions

git-svn-id: http://svn.code.sf.net/p/nsis/code/NSIS@6500 212acab6-be3b-0410-9dea-997c60f758d6
Don't use _itot, only VC supports _itow?


git-svn-id: http://svn.code.sf.net/p/nsis/code/NSIS@6501 212acab6-be3b-0410-9dea-997c60f758d6
Fixed 5 GCC warnings


git-svn-id: http://svn.code.sf.net/p/nsis/code/NSIS@6502 212acab6-be3b-0410-9dea-997c60f758d6