Headless build fails due to missing X11 headers, if the build environment requires the use of --x-includes configure flag. The problem is the header file "src/java.desktop/share/native/libawt/java2d/pipe/Region.h" which is used in various places and includes "src/java.desktop/unix/native/common/awt/utility/rect.h" which defines a platform dependent type, which requires the X11 headers on linux.

In file included from /home/cgo/dev/openjdk/jdk/src/java.desktop/share/native/libawt/java2d/pipe/Region.h:34:0,
                 from /home/cgo/dev/openjdk/jdk/src/java.desktop/share/native/libawt/java2d/loops/Blit.c:27:
/home/cgo/dev/openjdk/jdk/src/java.desktop/unix/native/common/awt/utility/rect.h:32:10: fatal error: X11/Xlib.h: No such file or directory
 #include <X11/Xlib.h>
          ^~~~~~~~~~~~
compilation terminated.