As of Mavericks (OS X 10.9) the /usr/include directory is gone. Half of the answers here are obsolete, as the application will not compile until you sort out the include directory.
I solved the problem creating a symbolic link to MacOSX SDK in terminal, using the following command:
sudo ln -s /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.9.sdk/usr/include /usr/include
The application now compiles again.