set this build setting:

    C Language Dialect: C99 [-std=c99] 

Search the build settings for "c99" to find the setting. If it's not present as 
a build option, and if a compile error requires c99, then set the equivalent 
user-defined setting:

    GCC_C_LANGUAGE_STANDARD=c99 

For just the Debug configuration of your target, add this compiler definition 
to ensure that the library's debug-only code is included:

    Other C Flags: -DDEBUG=1 

Or, if the Other C Flags setting is not available in your target's build 
options, set the equivalent user-defined setting:

    OTHER_CFLAGS=-DDEBUG=1 

With the source files compiled directly in your project, refer directly to the 
headers by omitting the framework name, like

#import "GDataCalendar.h"

Notes for iPhone apps compiling the source files directly:

Be sure that the files GDataXMLNode.m and GDataXMLNode.h in the 
Common/Optional/XMLSupport group are included in your project. They are 
required for iPhone builds.

iPhone applications also need these build settings in the project or target:

    Header Search Paths: /usr/include/libxml2 

    Other Linker Flags: -lxml2