Fixed generating xcode4 projects with preprocessor definitions including quotes

--HG--
branch : macosx-quotes
Make table.isempty work with table which has a key as false
Merged in mologie/premake-stable/macosx-quotes (pull request #14)

Fixed generating xcode4 projects with preprocessor definitions including quotes
Merged in lzprgmr/premake-stable (pull request #17)

Make table.isempty work with table which has a key as false