USER_HEADER_SEARCH_PATHS added with -I, should use -iquote instead

[mstorsjo]

Include directories that are added from USER_HEADER_SEARCH_PATHS end up passed to the compiler with the -I flag, while xcodebuild uses -iquote. This means that even though these directories were only inteded for use with #include "foo.h", the also end up used for #include <foo.h>.